[objc_direct] also go through implementations when looking for clashes

Some methods are sometimes declared in the @implementation blocks which
can cause undiagnosed clashes.

Just write a checkObjCDirectMethodClashes() for this purpose.

Also make sure that "unavailable" selectors do not inherit
objc_direct_members.

Differential Revision: https://reviews.llvm.org/D76643


Signed-off-by: Pierre Habouzit <phabouzit@apple.com>
Radar-ID: rdar://problem/59332804, rdar://problem/59782963