[serialization] Stop recording the full paths to input source files.

There were no clients and it leaks information about the developer's system.
After this commit, there should be no full paths present in framework modules.
(App modules may contain search paths for debugging reasons, as well as a
full or relative path to the bridging header.)

Swift SVN r24851

lib/Serialization/ModuleFile.cpp

@@ -704,10 +704,6 @@ ModuleFile::ModuleFile(
         StringRef blobData;
         unsigned kind = cursor.readRecord(next.ID, scratch, &blobData);
         switch (kind) {
-        case input_block::SOURCE_FILE:
-          assert(scratch.empty());
-          SourcePaths.push_back(blobData);
-          break;
         case input_block::IMPORTED_MODULE: {
           bool exported, scoped;
           input_block::ImportedModuleLayout::readRecord(scratch,