Summary:
Upgrade gcc to 4.9.1 and clang to dev.
With new compilers I succeeded to run thread sanitizer, too. I'll post output (doesn't look good) and fix some things in separate diffs.
Test Plan: compiles with both g++ and clang
Reviewers: yhchiang, rven, sdong
Reviewed By: sdong
Subscribers: dhruba, leveldb
Differential Revision: https://reviews.facebook.net/D32007