[NFC] Move readAPValue/writeAPValue up the inheritance hierarchy

The implementation for (de)serialization of APValues can be shared
between Clang and Swift, so we prefer pushing the methods up
the inheritance hierarchy, instead of having the methods live in
ASTReader/ASTWriter. Fixes rdar://72592937.

Reviewed By: rjmccall

Differential Revision: https://reviews.llvm.org/D94196