propagate efWantStmt in semWhen (#23400)

fixes #23399

The new case introduced in #21657 is triggered by `efWantStmt` but the
`when` statement doesn't normally propagate this flag, so propagate it
when the `semCheck` param in `semWhen` is true which happens when the
`when` statement is `efWhenStmt` anyway.

(cherry picked from commit fb6c805568)

tests/modules/tmodulesymtype.nim

@@ -13,3 +13,10 @@ proc foo() =
     sequtils
 
 foo()
+
+# issue #23399
+when isMainModule:
+  sequtils #[tt.Error
+  ^ expression has no type: sequtils]#
+
+discard