AstGen: fix @export with undeclared identifier crashing

This required a third `if (found_already == null)` in another place in
AstGen.zig for this special case of `@export`.

Fixes #17188
4585cb1e2fWooster committed on 9/22/2023, 7:23:57 PM· committed by Andrew Kelleyparent3fc7413
3 files changedLine totals unavailable