[clang] Ensure --print-runtime-dir exits

Before this PR, `clang --print-runtime-dir` used to report a non-existant
directory if LLVM_ENABLE_PER_TARGET_RUNTIME_DIR=OFF.

We now check if any of the known runtime directories exist before
printing on stdout.

Author: aganea

clang/lib/Driver/Driver.cpp

@@ -2230,10 +2230,14 @@ bool Driver::HandleImmediateArgs(Compilation &C) {
   }
 
   if (C.getArgs().hasArg(options::OPT_print_runtime_dir)) {
-    if (std::optional<std::string> RuntimePath = TC.getRuntimePath())
-      llvm::outs() << *RuntimePath << '\n';
-    else
-      llvm::outs() << TC.getCompilerRTPath() << '\n';
+    for (auto RuntimePath :
+         {TC.getRuntimePath(), std::make_optional(TC.getCompilerRTPath())}) {
+      if (RuntimePath && getVFS().exists(*RuntimePath)) {
+        llvm::outs() << *RuntimePath << '\n';
+        return false;
+      }
+    }
+    llvm::outs() << "(runtime dir is not present)" << '\n';
     return false;
   }