[clang] [llvm] [NFC][SPIRV] Re-factor feature map initialisation for AMDGCN flavoured SPIR-V (PR #192067)

[Alex Voicu via cfe-commits]

https://github.com/AlexVlx commented:

@rampitec I did ping you about this some time ago.

https://github.com/llvm/llvm-project/pull/192067