This fixes a regression caused by delayed type definition searching
(#96755 and friends): If we end up adding a member (e.g. a typedef) to a
type that we've already attempted to complete (and failed), the
resulting AST would end up inconsistent (we would start to "forcibly"
complete it, but never finish it), and importing it into an expression
AST would crash.
This patch fixes this by detecting the situation and finishing the
definition as well.
(cherry picked from commit 57cd1000c9c93fd0e64352cfbc9fbbe5b8a8fcef)
c261de724b