[Support] Fix some warnings when self-hosting clang on Windows

llvm-svn: 186413

llvm/lib/Support/ErrorHandling.cpp

@@ -96,4 +96,9 @@ void llvm::llvm_unreachable_internal(const char *msg, const char *file,
     dbgs() << " at " << file << ":" << line;
   dbgs() << "!\n";
   abort();
+#ifdef LLVM_BUILTIN_UNREACHABLE
+  // Windows systems and possibly others don't declare abort() to be noreturn,
+  // so use the unreachable builtin to avoid a Clang self-host warning.
+  LLVM_BUILTIN_UNREACHABLE;
+#endif
 }