[clang] [clang][SPIR-V] Use the C calling convention as the target default (PR #210882)

[Arseniy Obolenskiy via cfe-commits]

aobolensk wrote:

> @aobolensk Could you please revise the title to something like "[clang][SPIR-V] Use the C calling convention as the target default"? Its impact is clearly wider than HIP.

Yes, done

https://github.com/llvm/llvm-project/pull/210882