Make Sema::CollectMultipleMethodsInGlobalPool() public.

It's useful for out-of-tree clients to be able to query the global
Objective-C method pool, and only Sema can do that right now.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@221744 91177308-0d34-0410-b5e6-96231b3b80d8