[Error] Make llvm::cantFail include the original error messages

Summary:
The current implementation eats the current errors and just outputs
the message parameter passed to llvm::cantFail.  This change appends
the original error message(s), so the user can see exactly why
cantFail failed.  New logic is conditional on NDEBUG.

Reviewed By: lhames

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D69057

llvm-svn: 375176