[clang] Add spirv-val compilation step when targeting SPIR-V (PR #188150)

[Steven Perron via cfe-commits]

https://github.com/s-perron approved this pull request.

The call to spirv-val looks good to me. The rest seems reasonable, but I'm not an expert in that area.

https://github.com/llvm/llvm-project/pull/188150