Expands the unofficial “feature” that allows extensions to define @objc methods declared in headers for the same module to also allow this for methods in the private clang module.
Fixes rdar://97810819.
Some mixed-language projects import Objective-C headers through their umbrella or bridging header that declare things that are actually implemented in Swift. This isn’t something we really supported or had tests for, but it happens in practice and we can’t break them.
Carve out a second exception to method conflict checking for when all of the conflicting methods are imported ObjC methods in either the same module or a bridging header.
Fixes rdar://96470068.
Let's try calling ObjCInterface::lookupMethod() instead. This also
eliminates an order dependency between selector conflict checking
and protocol member mirroring, which fixes a diagnostics regression
once lazy loading is enabled for mirrored protocol members.
