[analyzer] Fix filename in cross-file HTML report

[malcolm.parsons]

Summary

The filename is currently taken from the start of the path, while the
line and column are taken from the end of the path.
This didn't matter until cross-file path reporting was added.

[vlad.tsyrklevich]

This LGTM, but 1) you should add a test, probably just need to another CHECK to test/Coverage/html-multifile-diagnostics.c from D30406, and 2) someone from the analyzer team should sign off on it too.

[malcolm.parsons]

Add test

[george.karpenkov]

LGTM

[NoQ]

Thanks! I noticed the problem but never investigated it.

lib/StaticAnalyzer/Core/HTMLDiagnostics.cpp

if (SupportsCrossFileDiagnostics && FileIDs.size() > 1) {
     }
   }
 
   const RewriteBuffer *Buf = R.getRewriteBufferFor(FileIDs[0]);
   if (!Buf)
     return {};
 
   // Add CSS, header, and footer.
-  const FileEntry* Entry = SMgr.getFileEntryForID(FileIDs[0]);
+  FileID FID =
+      path.back()->getLocation().asLocation().getExpansionLoc().getFileID();
+  const FileEntry* Entry = SMgr.getFileEntryForID(FID);
   FinalizeHTML(D, R, SMgr, path, FileIDs[0], Entry, declName);
 
   std::string file;
   llvm::raw_string_ostream os(file);
   for (auto BI : *Buf)
     os << BI;
 
   return os.str();