[Driver] When forcing a crash print the bug report message

Commit a945037e moved the printing of the
"PLEASE submit a bug report" message to the crash handler, but that means we
don't print it when forcing a crash using FORCE_CLANG_DIAGNOSTICS_CRASH. Fix
this by adding a function to get the bug report message and printing it when
forcing a crash.

Differential Revision: https://reviews.llvm.org/D81672