[DebugInfo] Store optional DIFile::Source as pointer

getCanonicalMDString() also returns a nullptr for empty strings, which
tripped over the getSource() method. Solve the ambiguity of no source
versus an optional containing a nullptr by simply storing a pointer.

Differential Revision: https://reviews.llvm.org/D138658

llvm/lib/Bitcode/Writer/BitcodeWriter.cpp

@@ -1808,7 +1808,7 @@ void ModuleBitcodeWriter::writeDIFile(const DIFile *N,
   }
   auto Source = N->getRawSource();
   if (Source)
-    Record.push_back(VE.getMetadataOrNullID(*Source));
+    Record.push_back(VE.getMetadataOrNullID(Source));
 
   Stream.EmitRecord(bitc::METADATA_FILE, Record, Abbrev);
   Record.clear();