[clang] [HLSL][SPIRV] Fix calling convention for call in entry function. (PR #110275)

[Michal Paszkowski via cfe-commits]

michalpaszkowski wrote:

LGTM! I think changing the mangling for Vulkan is the best solution here.

One small comment, later on if/when you decide to use SPIRVBuiltins.cpp/.td for lowering builtins, there might be additional changes needed. The lookup function expects Itanium demangled strings.

https://github.com/llvm/llvm-project/pull/110275