Add new hidden option -print-on-crash that prints out IR that caused opt pipeline...

Add new hidden option -print-on-crash that prints out IR that caused opt pipeline to crash

A new hidden option -print-on-crash that prints the IR as it was upon entering
the last pass when there is a crash.

The IR is saved in its print form before each pass is started and a
signal handler is registered.  If the compilation crashes, the signal
handler will print the saved IR to dbgs().  This option
can be modified using -print-module-scope to get the IR for the complete
module.  Note that this option only works with the new pass manager.

Reviewed By: yrouban

Differential Revision: https://reviews.llvm.org/D86657