Driver: Restore -fkeep-inline-functions as an ignored flag

Several options were moved to the clang_ignored_gcc_optimization group
in r213365, but -fkeep-inline-functions was accidentally dropped. This
restores the flag.

Patch by Steven Wu. Thanks!

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@216522 91177308-0d34-0410-b5e6-96231b3b80d8