[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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