[llvm] [SPIR-V] Support `SPV_INTEL_int4` extension (PR #141031)

[Michal Paszkowski via llvm-commits]

https://github.com/michalpaszkowski commented:

Please also extend llvm/docs/SPIRVUsage.rst with a description of the new extension

https://github.com/llvm/llvm-project/pull/141031