Extracted from #144338.

This snippet used to work on 25.1 and prior versions

CREATE TABLE concur_exprs_tab (c1 int , c2 boolean);
CREATE UNIQUE INDEX concur_exprs_index_expr ON concur_exprs_tab ((c1::text COLLATE "C"));

but it doesn't anymore - we now get ERROR: invalid locale c: language: tag is not well-formed on the index creation. I bisected it to fb7a7b3.

Adding a GA-blocker for now so that we decide whether it's acceptable or not.

Jira issue: CRDB-49807