Support -fno-omit-frame-pointer with -pg.

Summary:
Previously, any instance of -fomit-frame-pointer would make it such that
-pg was an invalid flag combination. If -fno-omit-frame-pointer is
passed later on the command line (such that it actually takes effect),
-pg should be allowed.

Reviewers: nickdesaulniers

Reviewed By: nickdesaulniers

Subscribers: manojgupta, nickdesaulniers, cfe-commits, kongyi, chh, pirama

Differential Revision: https://reviews.llvm.org/D51713

llvm-svn: 342165