Modules: for ObjectiveC try to keep the definition invariant.

When deserializing ObjCInterfaceDecl with definition data, if we already have
a definition, try to keep the definition invariant; also pull in the
categories even if it is not what getDefinition returns (this effectively
combines categories).

rdar://27926200
rdar://26708823


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@281119 91177308-0d34-0410-b5e6-96231b3b80d8
22 files changed