[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