This patch does two things for readability:

- Use structured bindings.
- Remove extraneous .c_str().

Note that YamlIO.mapRequired takes StringRef as the key type.  As
such, we can implicitly construct StringRef from std::string.

io.mapRequired takes StringRef as the key type.  As such, we do not
need to create extraneous copies with str().c_str() or str().

Identified with readability-redundant-string-cstr.

Note that "override" makes "virtual" redundant.

Identified with modernize-use-override.

Use the nonstatic member instead.

I'm pretty sure the code in SPRIV is a layering violation. MC layer
files are using a CodeGen header.

Fixes #85578, a use-after-free caused by some `MCSymbolWasm` data being
freed too early.

Previously, `WebAssemblyAsmParser` owned the data that is moved to
`MCContext` by this PR, which caused problems when handling module ASM,
because the ASM parser was destroyed after parsing the module ASM, but
the symbols persisted.

The added test passes locally with an LLVM build with AddressSanitizer
enabled.

Implementation notes:

* I've called the added method
<code>allocate<b><i>Generic</i></b>String</code> and added the second
paragraph of its documentation to maybe guide people a bit on when to
use this method (based on my (limited) understanding of the `MCContext`
class). We could also just call it `allocateString` and remove that
second paragraph.
* The added `createWasmSignature` method does not support taking the
return and parameter types as arguments: Specifying them afterwards is
barely any longer and prevents them from being accidentally specified in
the wrong order.
* This removes a _"TODO: Do the uniquing of Signatures here instead of
ObjectFileWriter?"_ since the field it's attached to is also removed.
Let me know if you think that TODO should be preserved somewhere.

`ArrayRef` is supposed to be passed by value:

https://github.com/llvm/llvm-project/blob/bda05140fb758156ae35f3919e6283d94bfa3905/llvm/include/llvm/ADT/ArrayRef.h#L38-L39

Move WebAssemblyUtilities from Utils to the CodeGen library. It
primarily deals in MIR layer types, so it really lives in the CodeGen
library.

Move a variety of other things around to try create better separation.

See issue #64166 for more info on layering.

Move llvm/include/CodeGen/WasmAddressSpaces.h back to
llvm/lib/Target/WebAssembly/Utils.

Differential Revision: https://reviews.llvm.org/D156472

This fixes what I consider to be an API flaw I've tripped over
multiple times. The point this is constructed isn't well defined, so
depending on where this is first called, you can conclude different
information based on the MachineFunction. For example, the AMDGPU
implementation inspected the MachineFrameInfo on construction for the
stack objects and if the frame has calls. This kind of worked in
SelectionDAG which visited all allocas up front, but broke in
GlobalISel which hasn't visited any of the IR when arguments are
lowered.

I've run into similar problems before with the MIR parser and trying
to make use of other MachineFunction fields, so I think it's best to
just categorically disallow dependency on the MachineFunction state in
the constructor and to always construct this at the same time as the
MachineFunction itself.

A missing feature I still could use is a way to access an custom
analysis pass on the IR here.