[llvm] [clang] [LLVM] Add file magic detection for SPIR-V files. (PR #75363)

[Joseph Huber via cfe-commits]

jhuber6 wrote:

Added a test, for whatever reason I had to do a completely clean build to get the test to correctly pick up my changes to `Magic.cpp`, don't know why.

https://github.com/llvm/llvm-project/pull/75363