Use this one little trick to prevent optimizers from removing an
intentional stack overflow.


git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@197849 91177308-0d34-0410-b5e6-96231b3b80d8
Assert that tag decls are never marked (in)valid after definition is complete

Sema relies on this assumption.

Follow-up to r197848.

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@197850 91177308-0d34-0410-b5e6-96231b3b80d8