[HLSL] Support register binding attribute on global variable

[python3kgae]

Summary

Allow register binding attribute on variables.

Report warning when register binding attribute applies to local variable or static variable.
It will be ignored in this case.

Type check for register binding is tracked with https://github.com/llvm/llvm-project/issues/57886.

[python3kgae]

Remove empty line.

[aaron.ballman]

clang/include/clang/Parse/Parser.h

2884–2885

Let's assert we're in HLSL mode here so that we don't accidentally call this from non-HLSL parsing contexts.

clang/test/SemaHLSL/resource_binding_attr_error.hlsl

44

51

Let's add another test for a local extern, like:

void func() {
  extern RWBuffer<float> U : register(u5);
}

[python3kgae]

Ignore local extern decl when check ExternalGlobalVar

clang/test/SemaHLSL/resource_binding_attr_error.hlsl

51

Nice catch.
Never know you could do this.

[aaron.ballman]

clang/include/clang/Parse/Parser.h

2884–2885

Precommit CI finds a way to trigger this assert, so it looks like we caught a case we didn't know about (yay asserts!).

[python3kgae]

Limit MaybeParseHLSLSemantics to HLSL.

[python3kgae]

clang/include/clang/Parse/Parser.h

2884–2885

Yeah. Pre-commit CI is really cool. :)

It would be nice if there's an optional config so we can let it test experimental targets too.

[aaron.ballman]

LGTM!