[llvm] Reapply "[NFC][AMDGPU] Introduce AMDGPU::FlatVariant enum to replace uint64_t FLAT discriminant" (PR #200431)

[Valery Pykhtin via llvm-commits]

vpykhtin wrote:

I should have been added authors of original FlatVariant vairable as reviewers.

@Flakebi, @rovka I'm trying to find better name for the enum that would replace raw TSFlags usage in FlatVariant, could you please make a suggestion or justify current FlatAddrSpace (and move it to the defs of other addr space per @shiltian suggestion)?

https://github.com/llvm/llvm-project/pull/200431