[NFC] Doc and style fixes for #filePath implementation

This addresses some late-breaking review comments left by @hamishknight on apple/swift#29412.

lib/AST/Module.cpp

@@ -1964,7 +1964,7 @@ SourceFile::getInfoForUsedFilePaths() const {
         getASTContext().SourceMgr.getLocForBufferStart(BufferID);
   }
 
-  for (auto &vpath : VirtualFilenames) {
+  for (auto &vpath : VirtualFilePaths) {
     result[vpath.Item].virtualFileLocs.insert(vpath.Loc);
   }