Fix BackendConsumer::EmitOptimizationMessage()

Patch from Geoff Berry <gberry@codeaurora.org>

Fix BackendConsumer::EmitOptimizationMessage() to check if the
DiagnosticInfoOptimizationBase object has a valid location before
calling getLocation() to avoid dereferencing a null pointer inside
getLocation() when no debug info is present.

llvm-svn: 236898