Extend ExternalASTSource with the ability to lazily complete the

definition of an Objective-C class. Unlike with C/C++ classes, we
don't have a well-defined point in Sema where Objective-C classes are
checked for completeness, nor do we need to involve Sema when
completing a class. Therefore, we take the appropriate of having the
external AST source mark a particular Objective-C class as having an
external declaration; when using one of the accessors of an
Objective-C class that has an external declaration, we request that
the external AST source fill in the Objective-C class definition.

llvm-svn: 120627