Fix a problem in ASTContext::addRecordToClass handling forward declarations.

In a case like:

@class foo;
foo *P;

addRecordToClass was making an empty shadow struct for the foo interface and
completing it.  Later when an:

@interface foo
...
@endif

foo *Q;

was seen, ASTContext::addRecordToClass would think that foo was already laid
out and not lay out the definition.  This fixes it to create a forward declared
struct the first time around, then complete it when the definition is seen.

Note that this causes two tests to regress, because something is trying to get
the size of the forward declared structs returned by this.  Previously, this
would end up getting a size of zero but now it properly dies.  I'm not sure
what the right solution is for this, so I xfailed the tests.

Fariborz, please take a look at this.  The testcase in rdar://6676794 now gets
farther, but dies later because the objc ivar is not assigned a field number.

As an aside, I really don't like the fact that the objc front-end is creating
shadow C structs for ObjC types.  This seems like an implementation detail of
the code generator that could be fixed by better factoring of the extant code.

llvm-svn: 68106