[clang] [HLSL] Change default linkage of HLSL functions and `groupshared` variables (PR #93336)

[Eli Friedman via cfe-commits]

efriedma-quic wrote:

I'm not sure messing with the linkage this way will interact well with various C++ features.  In particular, there are various places that check the "language linkage", which you're not modifying: from the perspective of anything outside of codegen, these 
functions are just external.  So other parts of the code won't be aware of the correct linkage.  Off the top of my head. this affects mangling, the linkage of template functions, and various warnings.

I'm not sure how much we can do in terms of actually solving this, given the way clang is architected, but we should try to come up with some sort of plan going forward.

https://github.com/llvm/llvm-project/pull/93336