[llvm] simplify and clean-up DemangleConfig.h (#149163)

## Purpose
Simplify `DEMANGLE_` macro definitions in
`llvm/Demangle/DemangleConfig.h` for clarity/maintainability.

## Overview
* Alias `DEMANGLE_DUMP_METHOD`, `DEMANGLE_FALLTHROUGH`, and
`DEMANGLE_UNREACHABLE` macros to their `LLVM_` counterparts defined in
`llvm/Support/Compiler.h`
* Remove several `DEMANGLE_`-prefixed macros that were only used within
`DemangleConfig.h`

## Background
* It should be safe for the `Demangle` component library to depend on
`Support`, so there is no need for it to maintain copies of macros
defined in `llvm/Support/Compiler.h`.
* Since the canonical copy `llvm/Demangle/ItaniumDemangle.h` lives under
`libcxxabi`, it cannot directly reference the `LLVM_`-prefixed macros so
we define `DEMANGLE_`-prefixed aliases.

## Validation
* Built llvm-project on Windows with `clang-cl` and MSVC `cl`.
* Built llvm-project on Linux with `clang` and `gcc`.

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