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

[Nathan Gauër via cfe-commits]

Keenuts wrote:

> My only concern is that the parser does not allow digits in the middle of the semantic name. I believe that is possible for user semantics. It works for DXC. Otherwise LGTM.

Thank you all!
reworked the semantic parsing to allow digits in the middle of the identifier. This simplified a bit the parsing. Extended testing to cover those.
Also addressed all other PR comments.

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