[WebAssembly] Add WebAssembly::Specifier
Move wasm-specific members outside of MCSymbolRefExpr::VariantKind (a
legacy interface I am eliminating). Most changes are mechanic and
similar to what I've done for many ELF targets (e.g. X86 #132149)
Notes:
* `fixSymbolsInTLSFixups` is replaced with `setTLS` in
`WebAssemblyWasmObjectWriter::getRelocType`, similar to what I've done
for many ELF targets.
* `SymA->setUsedInGOT()` in `recordRelocation` is moved to
`getRelocType`.
While here, rename "Modifier' to "Specifier":
> "Relocation modifier", though concise, suggests adjustments happen during the linker's relocation step rather than the assembler's expression evaluation. I landed on "relocation specifier" as the winner. It's clear, aligns with Arm and IBM’s usage, and fits the assembler's role seamlessly.
Pull Request: https://github.com/llvm/llvm-project/pull/133116
