Avoid accessing an invalid PresumedLoc.

DiagnosticNoteRenderer asserts trying to emit its "while building
module Foo imported from bar.h:5" note when the presumed location
of the import is invalid. This assertion was added in r267914,
where most uses of 'getFilename' were updated to test 'isValid'
instead. This one must have been missed.

I can't come up with a test because this location is always valid
in C-based code, but external clients that manually import modules
(*cough*Swift*cough*) sometimes provide invalid SourceLocations.

rdar://problem/26099576

http://reviews.llvm.org/D21111

llvm-svn: 273976