[clang] Enabling Intel GPU Integration. (PR #65539)

[Sang Ik Lee via cfe-commits]

silee2 wrote:

> I suggest to extract `mgpu` interface changes and `serializetoSpirv` pass to 2 separate PRs.

Agree. And the changes are originally from different authors so should be splitted.

https://github.com/llvm/llvm-project/pull/65539