Prepare for removing PPCMCExpr. Adopt the newer naming convention with
AMDGPU/WebAssembly/VE/M68k.

Most changes are mechanic, except:

* ELFObjectWriter::shouldRelocateWithSymbol: .TOC.@tocbase does not
  register the undefined symbol.  Move the handling into the
  Sym->isUndefined() code path.
* ELFObjectWriter::fixSymbolsInTLSFixups's VK_PPC* cases are moved to
  PPCELFObjectWriter::getRelocType. We should do similar refactoring
  for other targets and eventually remove fixSymbolsInTLSFixups.

In the future, we should classify PPCMCExpr similar to AArch64MCExpr.

Identified with misc-include-cleaner.

This patch provides two major changes:

1. Add getRelocationInfo to check if a constant will have static, dynamic, or
   no relocations. (Also rename the original needsRelocation to needsDynamicRelocation.)
2. Only allow a constant with no relocations (static or dynamic) to be placed
   in a mergeable section.

This will allow unused symbols that contain static relocations and happen to
fit in mergeable constant sections (.rodata.cstN) to instead be placed in
unique-named sections if -fdata-sections is used and subsequently garbage collected
by --gc-sections.

See https://lists.llvm.org/pipermail/llvm-dev/2021-February/148281.html.

Differential Revision: https://reviews.llvm.org/D95960

Replace with forward declarations and move includes down to source files where required.

I also needed to move the TargetLoweringObjectFile::SectionForGlobal wrapper implementation down into TargetLoweringObjectFile.cpp

-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.

Currently -fuse-init-array option is not effective when target triple
does not specify os, on x86,x86_64.
i.e.

// -fuse-init-array is not honored.
$ clang -target i386 -fuse-init-array test.c -S

// -fuse-init-array is honored.
$ clang -target i386-linux -fuse-init-array test.c -S

This patch fixes first case.
And does cleanup.

Reviewers: rnk, craig.topper, fhahn, echristo

Reviewed By: rnk

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