[clang] [HLSL] Rewrite semantics parsing (PR #152537)

[Michał Górny via cfe-commits]

mgorny wrote:

I'm guessing this change broke the build for me:

```
FAILED: [code=1] docs/AttributeReference.rst /var/tmp/portage/llvm-core/clang-22.0.0.9999/work/x/y/clang-.arm64/docs/AttributeReference
.rst 
cd /var/tmp/portage/llvm-core/clang-22.0.0.9999/work/x/y/clang-.arm64/docs && /var/tmp/portage/llvm-core/clang-22.0.0.9999/work/x/y/cla
ng-.arm64/bin/clang-tblgen -gen-attr-docs -I/var/tmp/portage/llvm-core/clang-22.0.0.9999/work/clang/docs -I/var/tmp/portage/llvm-core/c
lang-22.0.0.9999/work/clang/include -I/var/tmp/portage/llvm-core/clang-22.0.0.9999/work/x/y/clang-.arm64/include -I/usr/lib/llvm/22/inc
lude -I/var/tmp/portage/llvm-core/clang-22.0.0.9999/work/clang/docs/../include/clang/Basic /var/tmp/portage/llvm-core/clang-22.0.0.9999
/work/clang/docs/../include/clang/Basic/Attr.td --write-if-changed -o AttributeReference.rst -d AttributeReference.rst.d && /usr/bin/cm
ake -E cmake_transform_depfile Ninja gccdepfile /var/tmp/portage/llvm-core/clang-22.0.0.9999/work/clang /var/tmp/portage/llvm-core/clan
g-22.0.0.9999/work/clang/docs /var/tmp/portage/llvm-core/clang-22.0.0.9999/work/x/y/clang-.arm64 /var/tmp/portage/llvm-core/clang-22.0.
0.9999/work/x/y/clang-.arm64/docs /var/tmp/portage/llvm-core/clang-22.0.0.9999/work/x/y/clang-.arm64/docs/AttributeReference.rst.d /var
/tmp/portage/llvm-core/clang-22.0.0.9999/work/x/y/clang-.arm64/CMakeFiles/d/63214aa3df9ce54c496015feb0fb3942e9cf73781c78cb807e54fac358a
06d47.d
/var/tmp/portage/llvm-core/clang-22.0.0.9999/work/clang/docs/../include/clang/Basic/Attr.td:4986:5: error: Attribute has no supported spellings; cannot be documented
def HLSLSV_DispatchThreadID : HLSLSemanticAttr</* Indexable= */ 0> {
    ^
```

https://github.com/llvm/llvm-project/pull/152537