[clang][cli] Don't always emit -f[no-]legacy-pass-manager

We don't need to always generate `-f[no-]experimental-new-pass-manager`.

This patch does not change the behavior of any other command line flag. (For example `-triple` is still being always generated.)

Reviewed By: dexonsmith, Bigcheese

Differential Revision: https://reviews.llvm.org/D92857