[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.

llvm-svn: 185740