Assign collations in partition bound expressions.

Failure to do this can result in errors during evaluation of
the bound expression, as illustrated by the new regression test.

Back-patch to v12 where the ability for partition bounds to be
expressions was added.

Discussion: https://postgr.es/m/CAJV4CdrZ5mKuaEsRSbLf2URQ3h6iMtKD=hik8MaF5WwdmC9uZw@mail.gmail.com

src/backend/parser/parse_utilcmd.c

@@ -4239,7 +4239,10 @@ transformPartitionBoundValue(ParseState *pstate, Node *val,
 
 	/* Simplify the expression, in case we had a coercion */
 	if (!IsA(value, Const))
+	{
+		assign_expr_collations(pstate, value);
 		value = (Node *) expression_planner((Expr *) value);
+	}
 
 	/*
 	 * transformExpr() should have already rejected column references,