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

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

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@375176 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed