MC: Don't crash after issuing a diagnostic.

MC: Don't crash after issuing a diagnostic.

Crashing is bad, m'kay? Fixing a 4 year old bug of my own creation.
Adding the testcase now which I should have added then which would have
long since caught this.

The problem is that printMessage() will display the diagnostic but not
set HadError to true, resulting in the assembler continuing on its way
and trying to create relocations for things that may not allow them or
otherwise get itself into trouble. Using the Error() helper function
here rather than calling printMessage() directly resolves this.

rdar://23133240

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@250557 91177308-0d34-0410-b5e6-96231b3b80d8