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.