[clang] [clang][SPIR-V] Use the SPIR-V backend by default (PR #129545)

[Joseph Huber via cfe-commits]

https://github.com/jhuber6 approved this pull request.

LG from the clang driver side. I'll let the SPIR-V experts determine the rest.

https://github.com/llvm/llvm-project/pull/129545