Move target-specific members outside of MCSymbolRefExpr::VariantKind
(a legacy interface I am eliminating). Most changes are mechanic,
except:

* ELFObjectWriter::shouldRelocateWithSymbol
* The legacy generic code uses `ELFObjectWriter::fixSymbolsInTLSFixups`
  to set `STT_TLS` (and use an unnecessary expression walk). The better
  way is to do this in `getRelocType`, which I have done for
  AArch64, PowerPC, and RISC-V.

In the future, we should encode expressions with a relocation specifier
as X86MCExpr and use MCValue::RefKind to hold the specifier of the
relocatable expression.
https://maskray.me/blog/2025-03-16-relocation-generation-in-assemblers

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/132149

32-bit x86 doesn't have an appropriate relocation type we can use to
elide the RTTI proxies, so we need to emit them. This would previously
cause crashes when using the relative vtable ABI for 32-bit x86.

There's an existing check in LLVM that can replace an offset to a
DSO-local symbol that is just a pointer to another symbol with a
GOTPCREL reloc. This is exactly what the RTTI proxy in the relative
vtables ABI is. This feature is supported for different macho platforms,
but not for ELF. This extends that support.

We already include TargetLoweringObjectFileImpl.h which includes it and we only use its types as part of TargetLoweringObjectFile* overridden methods.

-fuse-init-array is now the CC1 default but TargetLoweringObjectFileELF::UseInitArray still defaults to false.
The following two unknown OS target triples continue using .ctors/.dtors because InitializeELF is not called.

clang -target i386 -c a.c
clang -target x86_64 -c a.c

This cleanup fixes this as a bonus.

X86SpeculativeLoadHardeningPass::tracePredStateThroughCall can call
MCContext::createTempSymbol before TargetLoweringObjectFileELF::Initialize().
We need to call TargetLoweringObjectFileELF::Initialize() ealier.

test/CodeGen/X86/speculative-load-hardening-indirect.ll

Differential Revision: https://reviews.llvm.org/D71360

as it's causing msan failures.

This reverts commit 7899fe9da8d8df6f19ddcbbb877ea124d711c54b.

UseInitArray is now the CC1 default but TargetLoweringObjectFileELF::UseInitArray still defaults to false.
The following two unknown OS target triples continue using .ctors/.dtors because InitializeELF is not called.

clang -target i386 -c a.c
clang -target x86_64 -c a.c

This cleanup fixes this as a bonus.

Differential Revision: https://reviews.llvm.org/D71360

This reverts commit aa5ee8f244441a8ea103a7e0ed8b6f3e74454516.

This change broke the sanitizer buildbots. See comments at the patchset
(https://reviews.llvm.org/D71360) for more information.