... due to their close relationship. MCSection's inline functions (e.g.
iterator) access MCFragment, and we want MCFragment's inline functions
to access MCSection similarly (#146307).

Pull Request: https://github.com/llvm/llvm-project/pull/146315

to not expose SmallVector to the callers. We will make fixup storage out
of line.

Prepare for removing ARMMCExpr. Adopt the new naming convention (S_
instead of VK_; the relocation specifier was previously named
`VariantKind`)) used by most other targets.

Make ARMMCAsmInfo.h include ARMMCExpr.h and change .cpp files to include
ARMMCAsmInfo.h. We will eventually remove ARMMCExpr.h.

Remove FK_PCRel_* kinds from the generic fixup list, as they are not
generic like FK_Data_*. In getRelocType, FK_PCRel_* can be replaced with
FK_Data_* by leveraging the IsPCRel argument. Their inclusion in the
generic kind list caused confusion for PowerPC, RISCV, and VE targets.

The X86/M68k uses can be implemented as target-specific fixups.

The .syntax unified directive and .codeX/.code X directives are, other
than some simple common printing code, exclusively implemented in the
targets themselves. Thus, remove the corresponding MCAF_* flags and
reimplement the directives solely within the targets. This avoids
exposing all targets to all other targets' flags.

Since MCAF_SubsectionsViaSymbols is all that remains, convert it to its
own function like other directives, simplifying its implementation.

Note that, on X86, we now always need a target streamer when parsing
assembly, as it's now used for directives that aren't COFF-specific. It
still does not however need to do anything when producing a non-COFF
object file, so this commit does not introduce any new target streamers.

There is some churn in test output, and corresponding UTC regex changes,
due to comments no longer being flushed by these various directives (and
EmitEOL is not exposed outside MCAsmStreamer.cpp so we couldn't do so
even if we wanted to), but that was a bit odd to be doing anyway.

This is motivated by Morello LLVM, which adds yet another assembler flag
to distinguish A64 and C64 instruction sets, but did not update every
switch and so emits warnings during the build. Rather than fix those
warnings it seems better to instead make the problem not exist in the
first place via this change.

Armv8.1-M Mainline architecture is supported by Clang's driver & LLVM's
ARM backend, but MC would report an 'Unknown Arch: armv8.1-m.main' error
when processing '.arch armv8.1-m.main' assembler directives.

Similar to previous migration done for other targets (PowerPC, X86,
etc).

Note: ARMELFObjectWriter::needsRelocateWithSymbol is conservative and
already includes most specifiers.