Move the definition-specific data of ObjCInterfaceDecl into a

Move the definition-specific data of ObjCInterfaceDecl into a
separately-allocated DefinitionData structure, which we manage the
same way as CXXRecordDecl::DefinitionData. This prepares the way for
making ObjCInterfaceDecls redeclarable, to more accurately model
forward declarations of Objective-C classes and eliminate the mutation
of ObjCInterfaceDecl that causes us serious trouble in the AST reader.

Note that ObjCInterfaceDecl's accessors are fairly robust against
being applied to forward declarations, because Clang (and Sema in
particular) doesn't perform RequireCompleteType/hasDefinition() checks
everywhere it has to. Each of these overly-robust cases is marked with
a FIXME, which we can tackle over time.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@146644 91177308-0d34-0410-b5e6-96231b3b80d8