The object file format specific derived classes are used in context
where the type is statically known. We don't use isa/dyn_cast and we
want to eliminate MCSymbol::Kind in the base class.

This patch fixes:

  llvm/lib/Target/WebAssembly/WebAssemblyISelDAGToDAG.cpp:126:26:
  error: lambda capture 'DAG' is not used
  [-Werror,-Wunused-lambda-capture]

  llvm/lib/Target/WebAssembly/WebAssemblyMCInstLower.cpp:239:28:
  error: unused variable 'Info' [-Werror,-Wunused-variable]

This adds an llvm intrinsic for WebAssembly to test the type of a
function. It is intended for adding a future clang builtin
` __builtin_wasm_test_function_pointer_signature` so we can test whether
calling a function pointer will fail with function signature mismatch.

Since the type of a function pointer is just `ptr` we can't figure out
the expected type from that.
The way I figured out to encode the type was by passing 0's of the
appropriate type to the intrinsic.
The first argument gives the expected type of the return type and the
later values give the expected
type of the arguments. So
```llvm
@llvm.wasm.ref.test.func(ptr %func, float 0.000000e+00, double 0.000000e+00, i32 0)
```
tests if `%func` is of type `(double, i32) -> (i32)`. It will lower to:
```wat
local.get $func
table.get $__indirect_function_table
ref.test (double, i32) -> (i32)
```
To indicate the function should be void, I somewhat arbitrarily picked
`token poison`, so the following tests for `(i32) -> ()`:
```llvm
@llvm.wasm.ref.test.func(ptr %func, token poison, i32 0)
```

To lower this intrinsic, we need some place to put the type information.
With `encodeFunctionSignature()` we encode the signature information
into an `APInt`. We decode it in `lowerEncodedFunctionSignature` in
`WebAssemblyMCInstLower.cpp`.

Currently wasm adds an extra level of options that work backwards
from the standard options, and overwrites them. The ExceptionModel
field in TM->Options is the standard user configuration option for the
exception model to use. MCAsmInfo's ExceptionsType is a constant for the
default to use for the triple if not explicitly set in the TargetOptions
ExceptionModel. This was adding 2 custom flags, changing the MCAsmInfo
default, and overwriting the ExceptionModel from the custom flags.

These comments about compiling bitcode with clang are describing a
toolchain
bug or user error. TargetOptions is bad, and we should move to
eliminating it.
It is module state not captured in the IR. Ideally the exception model
should either
come implied from the triple, or a module flag and not depend on this
side state.
Currently it is the responsibility of the toolchain and/or user to
ensure the same
command line flags are used at each phase of the compilation. It is not
the backend's
responsibilty to try to second guess these options.

-wasm-enable-eh and -wasm-enable-sjlj should also be removed in favor of
the standard
exception control. I'm a bit confused by how all of these fields are
supposed to interact,
but there are a few uses in the backend that are directly looking at
these flags instead
of the already parsed ExceptionModel which need to be cleaned up.

Additionally, this was enforcing some rules about the combinations of
flags at a random
point in the IR pass pipeline configuration. This is a module property
that should
be handled at TargetMachine construction time at the latest. This
required adding flags
to a few mir and clang tests which never got this far to avoid hitting
the errors.

to align with targets that have made the transition.

Move wasm-specific members outside of MCSymbolRefExpr::VariantKind (a
legacy interface I am eliminating). Most changes are mechanic and
similar to what I've done for many ELF targets (e.g. X86 #132149)

Notes:

* `fixSymbolsInTLSFixups` is replaced with `setTLS` in
  `WebAssemblyWasmObjectWriter::getRelocType`, similar to what I've done
  for many ELF targets.
* `SymA->setUsedInGOT()` in `recordRelocation` is moved to
  `getRelocType`.

While here, rename "Modifier' to "Specifier":

> "Relocation modifier", though concise, suggests adjustments happen during the linker's relocation step rather than the assembler's expression evaluation. I landed on "relocation specifier" as the winner. It's clear, aligns with Arm and IBM’s usage, and fits the assembler's role seamlessly.

Pull Request: https://github.com/llvm/llvm-project/pull/133116

#125983

`try_table`'s `catch` or `catch_ref`'s target block's return type should
be `i64` and `(i64, exnref)` in case of wasm64.

Identified with misc-include-cleaner.

This adds the basic assembly generation support for the final EH
proposal, which was newly adopted in Sep 2023 and advanced into Phase 4
in Jul 2024:

https://github.com/WebAssembly/exception-handling/blob/main/proposals/exception-handling/Exceptions.md

This adds support for the generation of new `try_table` and `throw_ref`
instruction in .s asesmbly format. This does NOT yet include
- Block annotation comment generation for .s format
- .o object file generation
- .s assembly parsing
- Type checking (AsmTypeCheck)
- Disassembler
- Fixing unwind mismatches in CFGStackify

These will be added as follow-up PRs.

---

The format for `TRY_TABLE`, both for `MachineInstr` and `MCInst`, is as
follows:
```
TRY_TABLE type number_of_catches catch_clauses*
```
where `catch_clause` is
```
catch_opcode tag+ destination
```
`catch_opcode` should be one of 0/1/2/3, which denotes
`CATCH`/`CATCH_REF`/`CATCH_ALL`/`CATCH_ALL_REF` respectively. (See
`BinaryFormat/Wasm.h`) `tag` exists when the catch is one of `CATCH` or
`CATCH_REF`.
The MIR format is printed as just the list of raw operands. The
(stack-based) assembly instruction supports pretty-printing, including
printing `catch` clauses by name, in InstPrinter.

In addition to the new instructions `TRY_TABLE` and `THROW_REF`, this
adds four pseudo instructions: `CATCH`, `CATCH_REF`, `CATCH_ALL`, and
`CATCH_ALL_REF`. These are pseudo instructions to simulate block return
values of `catch`, `catch_ref`, `catch_all`, `catch_all_ref` clauses in
`try_table` respectively, given that we don't support block return
values except for one case (`fixEndsAtEndOfFunction` in CFGStackify).
These will be omitted when we lower the instructions to `MCInst` at the
end.

LateEHPrepare now will have one more stage to covert
`CATCH`/`CATCH_ALL`s to `CATCH_REF`/`CATCH_ALL_REF`s when there is a
`RETHROW` to rethrow its exception. The pass also converts `RETHROW`s
into `THROW_REF`. Note that we still use `RETHROW` as an interim pseudo
instruction until we convert them to `THROW_REF` in LateEHPrepare.

CFGStackify has a new `placeTryTableMarker` function, which places
`try_table`/`end_try_table` markers with a necessary `catch` clause and
also `block`/`end_block` markers for the destination of the `catch`
clause.

In MCInstLower, now we need to support one more case for the multivalue
block signature (`catch_ref`'s destination's `(i32, exnref)` return
type).

InstPrinter has a new routine to print the `catch_list` type, which is
used to print `try_table` instructions.

The new test, `exception.ll`'s source is the same as
`exception-legacy.ll`, with the FileCheck expectations changed. One
difference is the commands in this file have `-wasm-enable-exnref` to
test the new format, and don't have `-wasm-disable-explicit-locals
-wasm-keep-registers`, because the new custom InstPrinter routine to
print `catch_list` only works for the stack-based instructions (`_S`),
and we can't use `-wasm-keep-registers` for them.

As in `exception-legacy.ll`, the FileCheck lines for the new tests do
not contain the whole program; they mostly contain only the control flow
instructions for readability.