Cleanup: Pass std::string arguments by reference (this avoids making
unnecessary copies) and label them const, per the
Google style guide.

Tested:
  $ ./bootstrap_test.sh all

--
Change-Id: I17636703cf43283f71b9b913c130a51065bd896b
Reviewed-on: https://bazel-review.googlesource.com/1350
MOS_MIGRATED_REVID=93876332
diff --git a/src/main/cpp/option_processor.h b/src/main/cpp/option_processor.h
index 9bc7a38..15724e8 100644
--- a/src/main/cpp/option_processor.h
+++ b/src/main/cpp/option_processor.h
@@ -94,7 +94,8 @@
     const int Index() const { return index_; }
 
    private:
-    static blaze_exit_code::ExitCode Parse(string filename, const int index,
+    static blaze_exit_code::ExitCode Parse(const string& filename,
+                                           const int index,
                                            std::vector<RcFile>* rcfiles,
                                            std::map<string,
                                            std::vector<RcOption> >* rcoptions,