Enhance the ObjC global method pool to record whether there were 0, 1, or >= 2 methods (with a particular selector) inside categories.

This is done by extending ObjCMethodList (which is only used by the global method pool) to have 2 extra bits of information.
We will later take advantage of this info in global method pool for the overridden methods calculation.
In ASTContext::getOverriddenMethods, call overridden_methods_begin/overridden_methods_end directly.

This avoids unnecessary Decl::getASTContext() invocations.
Use the extra info in global method pool to speed up looking for ObjC overridden methods.

When we are in a implementation, we check the global method pool whether there were category
methods with the same selector. If there were none (common case) we don't need to do lookups for
overridden methods again.

Note that for an interface method (if we don't encounter its implementation), it is considered that
it overrides methods that were declared before it, not for category methods introduced after it.

This is tradeoff in favor of performance, since it is expensive to do lookups in case there was a
category, and moving the global method pool to ASTContext (so we can check it) would increase complexity.

rdar://13508196