MC: don't create a backtrace for diagnostics.

For better or worse, this is currently the normal error reporting path
when dealing with backend errors from inline assembly. It's not just
internal compiler issues that come through here, so we shouldn't be
creating a backtrace on this path.

rdar://16329947

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@203979 91177308-0d34-0410-b5e6-96231b3b80d8