[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 |   };
      |   ^
```

clang/include/clang/Basic/TargetCXXABI.h

@@ -111,6 +111,7 @@ class TargetCXXABI {
     case Microsoft:
       return T.isKnownWindowsMSVCEnvironment();
     }
+    llvm_unreachable("invalid CXXABI kind");
   };
 
   /// Does this ABI generally fall into the Itanium family of ABIs?