Switch to Clang 3.6.

This change also defaults C compilations with clang to gnu99, and turns off
the -Winconsistent-missing-override flag, since we need further cleanup to
turn it on. -finline-functions is an unnecessary flag that only gcc needs.

Change-Id: I1cfcaedca32d9e3c248a6aa9460603205901c491
Merge "Switch to Clang 3.6."