[PCH] When completing an objc forward reference, do not serialize the chain of...

[PCH] When completing an objc forward reference, do not serialize the chain of its categories because
it is going to be rewritten (and the chain will be serialized again), otherwise we may form a cycle in its
categories list when deserializing.

Also introduce ASTMutationListener::CompletedObjCForwardRef to notify that a forward reference
was completed; using Decl's isChangedSinceDeserialization/setChangedSinceDeserialization
is bug inducing and kinda gross, we should phase it out.

Fixes infinite loop in rdar://10418538.

llvm-svn: 144465