When @compatibility_alias is used with an ObjC generic class, this ends up importing as a generic typealias. PrintAsObjC previously didn’t handle declarations involving these types correctly; it would fail an assertion in asserts compilers, and potentially print an incorrect compatibility header in non-asserts compilers.
This PR makes it so that PrintAsObjC can now correctly use generic typealiases imported from Objective-C modules. It is, of course, still not possible to declare a generic typealias in Swift that will be printed into the Objective-C header.
Fixes rdar://67256866.
For historic reasons, Clang's representation of an Objective-C class
declaration ObjCInterfaceDecl) and compatibility alias
(ObjCCompatibleAliasDecl) are not actually Clang TypeDecl nodes. Cope
with this in Objective-C printing, fixing rdar://problem/32308192.
