to align with targets that have made the transition.

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

Follow the X86 and Mips renaming.

> "Relocation modifier" suggests adjustments happen during the linker's relocation step rather than the assembler's expression evaluation.
> "Relocation specifier" is clear, aligns with Arm and IBM’s usage, and fits the assembler's role seamlessly.

In addition, rename *MCExpr::getKind, which confusingly shadows the base class getKind.

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.

Note that llvm::support::endianness has been renamed to
llvm::endianness while becoming an enum class as opposed to an
enum. This patch replaces support::{big,little,native} with
llvm::endianness::{big,little,native}.

Now that llvm::support::endianness has been renamed to
llvm::endianness, we can use the shorter form.  This patch replaces
support::endianness with llvm::endianness.

See issue #64166 for more information about the layering issue.

The PPCMCTargetDesc library was including CodeGen headers such as
PPCInstrInfo.h and calling inline functions in them. This doesn't work
in the Bazel build, and is error-prone. If the inline function moves to
a cpp file, it will result in linker errors.

To address the issue, I moved several inline functions to
PPCMCTargetDesc.cpp, and declared them in the PPC namespace in
PPCMCTargetDesc.h, which seemed like the most straightforward fix.

Differential Revision: https://reviews.llvm.org/D156488