AArch64: Replace AArch64MCExpr with MCSpecifierExpr
Replace AArch64MCExpr, which encodes expressions with relocation
specifiers, with the new generic MCSpecifierExpr interface, aligning
with other targets by phasing out target-specific XXXMCExpr classes.
Temporarily convert AArch64MCExpr to a namespace to avoid renaming
`AArch64MCExpr::VK_` constants in this PR. A follow-up patch will rename
these to `AArch64::S_` to match the convention used by other targets.
Move helper functions to AArch64MCAsmInfo.h, with the goal of eventually
removing AArch64MCExpr.h.
Pull Request: https://github.com/llvm/llvm-project/pull/144632
