[clang] NFC: Refactor custom class into a lambda in CompilerInvocation

Change `makeFlagToValueNormalizer` so that one specialization converts all integral/enum arguments into `uint64_t` and forwards them to the more generic version.

This makes it easy to replace the custom `FlagToValueNormalizer` struct with a lambda, which is the common approach in other (de)normalizers.

Finally, drop custom `is_int_convertbile` in favor of `llvm::is_integral_or_enum`.

Reviewed By: dexonsmith

Differential Revision: https://reviews.llvm.org/D93628