AstGen: make pointless discard error more strict

The error should only happen as a result of `_ = <expr>` not
for an operand of a break expression that is discarded.

Closes #13212
0a188190b3Veikka Tuominen committed on 11/11/2022, 3:59:53 PMparentd2cc551
2 files changedLine totals unavailable