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

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@273976 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Frontend/DiagnosticRenderer.cpp

@@ -618,7 +618,7 @@ DiagnosticNoteRenderer::emitBuildingModuleLocation(SourceLocation Loc,
   // Generate a note indicating the include location.
   SmallString<200> MessageStorage;
   llvm::raw_svector_ostream Message(MessageStorage);
-  if (PLoc.getFilename())
+  if (PLoc.isValid())
     Message << "while building module '" << ModuleName << "' imported from "
             << PLoc.getFilename() << ':' << PLoc.getLine() << ":";
   else