Several improvement to Sema::ParseRecordBody()...

- Adding a safer prologue. The previous prologue would accept a null and therefore assume we had an interface (which was incorrect).
- Fixed FieldDecl's classof method. This allowed me to simplify some unnecessary casting.
- When diagnosing errors, make sure the FieldDecl/EnclosingDecl are marked as invalid. In addition, don't delete the field...rather, add all fields to the enclosing decl. Memory management can/should be done elsewhere. This code was never "upgraded" to the recently added invalid decl strategy.

llvm-svn: 41964