[NFC][Clang][FMV] Refactor sema checking of target_version/clones attributes. (#149067)
Sema currently has checkTargetVersionAttr and
checkTargetClonesAttrString to diagnose the said attributes. However the
code tries to handle all of AArch64, RISC-V and X86 targets at once
which is hard to maintain, therefore I am splitting these functions.
Unfortunately I could not use polymorphism because all of Sema, SemaARM,
SemaRISCV and SemaX86 inherit from SemaBase. The Sema instance itself
contains instances of every other target specific Sema.