[clang][cli] Accept strings instead of options in ImpliedByAnyOf

To be able to refer to constant keypaths (e.g. `defvar cplusplus = LangOpts<"CPlusPlus">`) inside `ImpliedByAnyOf`, let's accept strings instead of `Option` instances.

This somewhat weakens the guarantees that we're referring to an existing (option) record, but we can still use the option.KeyPath syntax to simulate this.

Reviewed By: dexonsmith

Differential Revision: https://reviews.llvm.org/D95344