getImm() already returns int64_t.

MCFixup::Loc has been removed in favor of MCExpr::Loc through
`const MCExpr *Value` (commit 777391a2164b89d2030ca013562151ca3c3676d1).

Identified with misc-include-cleaner.

This adds support for binary generation for the new EH proposal.

So far the only case that we emitted variable immediate operands in
binary has been `br_table`'s destinations. (Other `variable_ops` uses in
TableGen files are register operands, such as the operands of `call`, so
they don't get emitted in binary as a part of the same instruction.)

With this PR, variable immediate operands can include `try_table`'s
operands:
- The number of of catch clauses
- catch clauses sub-opcodes
  - `catch`: 0x00
  - `catch_ref`: 0x01
  - `catch_all`: 0x02
  - `catch_all_ref`: 0x03
- catch clauses' destinations

With `try_table`, we now have variable expr operands for `try_table`'s
catch clauses' tags. We treat their fixups in the same way we do for
tags in other instructions such as in `throw`.

Diff without whitespace will be easier to view.

Note that llvm::support::endianness has been renamed to
llvm::endianness while becoming an enum class as opposed to an
enum. This patch replaces support::{big,little,native} with
llvm::endianness::{big,little,native}.

For inline WebAssembly, passing a numeric operand to global.get is
unsupported. This causes encodeInstruction to reach an llvm_unreachable
call, leading to undefined behaviors. This patch fixes the issue for
this invalid instruction encoding, making it report an error by adding
an MCContext field in class WebAssemblyMCCodeEmitter.

Reviewed By: sbc100, bryanpkc

Differential Revision: https://reviews.llvm.org/D154734

Change MCInstrDesc::operands to return an ArrayRef so we can easily use
it everywhere instead of the (IMHO ugly) opInfo_begin and opInfo_end.
A future patch will remove opInfo_begin and opInfo_end.

Also use it instead of raw access to the OpInfo pointer. A future patch
will remove this pointer.

Differential Revision: https://reviews.llvm.org/D142213