clang: Don't warn on unused momit-leaf-frame-pointer when frame pointers are off.

This fixes a regression from r365860: As that commit message
states, there are 3 valid states targeted by the combination of
-f(no-)omit-frame-pointer and -m(no-)omit-leaf-frame-pointer.

After r365860 it's impossible to get from state 10 (omit just
leaf frame pointers) to state 11 (omit all frame pointers)
in a single command line without getting a warning.

This change restores that functionality.

Fixes PR42966.

Differential Revision: https://reviews.llvm.org/D66142

llvm-svn: 368728