[clang][TargetCXXABI] Fix -Wreturn-type warning (NFC)

GCC warning:
```
In file included from /llvm-project/clang/include/clang/Basic/LangOptions.h:22,
                 from /llvm-project/clang/include/clang/Frontend/CompilerInvocation.h:16,
                 from /llvm-project/clang/lib/Frontend/CompilerInvocation.cpp:9:
/llvm-project/clang/include/clang/Basic/TargetCXXABI.h: In static member function ‘static bool clang::TargetCXXABI::isSupportedCXXABI(const llvm::Triple&, clang::TargetCXXABI::Kind)’:
/llvm-project/clang/include/clang/Basic/TargetCXXABI.h:114:3: warning: control reaches end of non-void function [-Wreturn-type]
  114 |   };
      |   ^
```