[objc-arc] Refactor runtime entrypoint declaration entrypoint creation.

[objc-arc] Refactor runtime entrypoint declaration entrypoint creation.

This is the first patch in a series of 3 patches which clean up how we create
runtime function declarations in the ARC optimizer when they do not exist
already in the IR.

Currently we have a bunch of duplicated code in ObjCARCOpts, ObjCARCContract
that does this. This patch refactors that code into a separate class called
ARCRuntimeEntryPoints which lazily creates the declarations for said
entrypoints.

The next two patches will consist of the work of refactoring
ObjCARCContract/ObjCARCOpts to use this new code.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@185740 91177308-0d34-0410-b5e6-96231b3b80d8