[AST][NFC] Silence GCC warning about broken strict aliasing rules

The deserialize() method would trigger the following warning on GCC <7:

   warning: dereferencing type-punned pointer will break
       strict-aliasing rules [-Wstrict-aliasing]

       ParamIdx P(*reinterpret_cast<ParamIdx *>(&S));
                                                  ^

&S was previously reinterpret_casted from a ParamIdx into a SerialType,
it is therefore safe to cast back into a ParamIdx. Similar to what was
done in D50608, we replace it with two static_cast via void * which
silences the warning and presumably makes GCC understand that no
strict-aliasing violation is happening.

No functional change intended.

Reviewed By: aaron.ballman

Differential Revision: https://reviews.llvm.org/D92384