a selector, the entry should be complete, containing everything introduced by
that module and all modules it imports.
Before writing out the method pool of a module, we sync up the out of date
selectors by pulling in methods for the selectors, from all modules it imports.
In ReadMethodPool, after pulling in the method pool entry for module A, this
lets us skip the modules that module A imports.
rdar://problem/25900131
git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@268091 91177308-0d34-0410-b5e6-96231b3b80d8
Manman Ren