Avoid accessing an invalid PresumedLoc

[jordan_rose]

Summary

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.

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

[jordan_rose]

*ping*

I'm happy to have someone else review this (or "LGTM" this), but it's so small that I'm not sure who else to ask. I'd rather not just commit it cause it's been a while since I've touched Clang.

[benlangmuir]

LGTM

[jordan_rose]

Committed as r273976.

lib/Frontend/DiagnosticRenderer.cpp

 void
 DiagnosticNoteRenderer::emitBuildingModuleLocation(SourceLocation Loc,
                                                    PresumedLoc PLoc,
                                                    StringRef ModuleName,
                                                    const SourceManager &SM) {
   // 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
     Message << "while building module '" << ModuleName << "':";
   emitNote(Loc, Message.str(), &SM);
 }