Add a new optimization option -Og

Summary:
Just like gcc, we should have the -Og option as more and more software are using it:
https://llvm.org/bugs/show_bug.cgi?id=20765

Reviewers: echristo, dberlin, dblaikie, keith.walker.arm, rengolin

Subscribers: aprantl, friss, mehdi_amini, RKSimon, probinson, majnemer, cfe-commits

Differential Revision: https://reviews.llvm.org/D24998

llvm-svn: 286602