[clang][NFC] refactor GlobalMethodPool to encapsulate its map
This refactor changes the GlobalMethodPool to a class that contains
the DenseMap of methods. This is to allow for the addition of a
separate DenseSet in a follow-up diff that will handle method
de-duplication when inserting methods into the global method pool.
Changes:
- the `GlobalMethods` pair becomes `GlobalMethodPool::Lists`
- the `GlobalMethodPool` becomes a class containing the `DenseMap` of methods
- pass through methods are added to maintain most of the existing code without changing `MethodPool` -> `MethodPool.Methods` everywhere
Reviewed By: dexonsmith
Differential Revision: https://reviews.llvm.org/D109898