Improve our semantic error recovery.

When an error made a record member invalid, the record would stay as "isBeingDefined" and
not "completeDefinition". Even easily recoverable errors ended up propagating records in
such "beingDefined" state, for example:

struct A {
  ~A() const; // expected-error {{'const' qualifier is not allowed on a destructor}}
};
struct B : A {}; // A & B would stay as "not complete definition" and "being defined".

This weird state was impending lookups in the records and hitting assertion in the ASTWriter.

Part of rdar://11007039

llvm-svn: 152432