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

[Steven Perron via cfe-commits]

s-perron wrote:

This PR does 2 things, so I will close this one and split it up. There is also some concern about adopting the Microsoft ABI, which needs to be discussed.

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