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