[clang] [HIP][SPIR-V] Enable SPIR-V backend by default (PR #198338)

[Dmitry Sidorov via cfe-commits]

MrSidims wrote:

> Thanks again @MrSidims. Before approving, it would be great to run a PSDB to make sure there are no obvious breaking changes in the stack.

Assuming the SPIR-V backend is enabled as an LLVM target - CI is clean.

https://github.com/llvm/llvm-project/pull/198338