Use MCSpecifierExpr directly and remove the ARMMCExpr subclass. Define
printImpl and evaluateAsRelocationImpl within ARM*MCAsmInfo classes.
While there is some duplication, it enables better separation for
object file formats.

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.

This annotates the `Twine` passed to the constructors of the various
DiagnosticInfo subclasses with `[[clang::lifetimebound]]`, which causes
us to warn when we would try to print the twine after it had already
been destructed.

We also update `DiagnosticInfoUnsupported` to hold a `const Twine &`
like all of the other DiagnosticInfo classes, since this warning allows
us to clean up all of the places where it was being used incorrectly.

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

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

This reverts commit 40986feda8b1437ed475b144d5b9a208b008782a.

Reapply with fix to prevent temporary Twine from going out of scope.

Reverts llvm/llvm-project#119485

Breaks builders, details in llvm/llvm-project#119485

Currently LLVMContext::emitError emits any error as an "inline asm"
error which does not make any sense. InlineAsm appears to be special,
in that it uses a "LocCookie" from srcloc metadata, which looks like
a parallel mechanism to ordinary source line locations. This meant
that other types of failures had degraded source information reported
when available.

Introduce some new generic error types, and only use inline asm
in the appropriate contexts. The DiagnosticInfo types are still
a bit of a mess, and I'm not sure why DiagnosticInfoWithLocationBase
exists instead of just having an optional DiagnosticLocation in the
base class.

DK_Generic is for any error that derives from an IR level instruction,
and thus can pull debug locations directly from it. DK_GenericWithLoc
is functionally the generic codegen error, since it does not depend
on the IR and instead can construct a DiagnosticLocation from the
MI debug location.

This allows lowering individual instructions in a bundle before a single
call to EmitToStreamer for VLIW targets.

[ARM] generate armv6m eXecute Only (XO) code for immediates, globals

Previously eXecute Only (XO) support was implemented for targets that support
MOVW/MOVT (~armv7+). See: https://reviews.llvm.org/D27449

XO prevents the compiler from generating data accesses to code sections. This
patch implements XO codegen for armv6-M, which does not support MOVW/MOVT, and
must resort to the following general pattern to avoid loads:

    movs    r3, :upper8_15:foo
    lsls    r3, #8
    adds    r3, :upper0_7:foo
    lsls    r3, #8
    adds    r3, :lower8_15:foo
    lsls    r3, #8
    adds    r3, :lower0_7:foo
    ldr     r3, [r3]

This is equivalent to the code pattern generated by GCC.

The above relocations are new to LLVM and have been implemented in a parent
patch: https://reviews.llvm.org/D149443.

This patch limits itself to implementing codegen for this pattern and enabling
XO for armv6-M in the backend.

Separate patches will follow for:
- switch tables
- replacing specific loads from constant islands which are spread out over the
  ARM backend codebase. Amongst others: FastISel, call lowering, stack frames.

Reviewed By: john.brawn

Differential Revision: https://reviews.llvm.org/D152795