[LLC] Add an inline assembly diagnostics handler.

[LLC] Add an inline assembly diagnostics handler.

Summary:
llc would hit a fatal error for errors in inline assembly. The
diagnostics message is now printed.

Reviewers: rengolin, MatzeB, javed.absar, anemet

Reviewed By: anemet

Subscribers: jyknight, nemanjai, llvm-commits

Differential Revision: https://reviews.llvm.org/D29408

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@293999 91177308-0d34-0410-b5e6-96231b3b80d8