Hint how to get a symbolized stack trace if llvm-symbolizer is not found on crashes

Most users of LLVM tools hit the raw traces and don't know how to get LLVM to
symbolize automatically for them.

When we print the non-symbolized stack trace, we will add information about
how to get it symbolized.

Reviewed By: MaskRay

Differential Revision: https://reviews.llvm.org/D88269

lib/Support/Unix/Signals.inc

@@ -576,6 +576,9 @@ void llvm::sys::PrintStackTrace(raw_ostream &OS, int Depth) {
     Depth = depth;
   if (printSymbolizedStackTrace(Argv0, StackTrace, Depth, OS))
     return;
+  OS << "Stack dump without symbol names (ensure you have llvm-symbolizer in "
+        "your PATH or set the environment var `LLVM_SYMBOLIZER_PATH` to point "
+        "to it):\n";
 #if HAVE_DLFCN_H && HAVE_DLADDR
   int width = 0;
   for (int i = 0; i < depth; ++i) {