Reflect normalization of query strings for utilities in pg_stat_statements

Reflect normalization of query strings for utilities in pg_stat_statements

Applying normalization changes how the following query strings are
reflected in pg_stat_statements, by showing Const nodes with a
dollar-signed parameter as this is how such queries are structured
internally once parsed:
- DECLARE
- EXPLAIN
- CREATE MATERIALIZED VIEW
- CREATE TABLE AS

More normalization could be done in the future depending on the parts
where query jumbling is applied (like A_Const nodes?), the changes being
reflected in the regression tests in majority created in de2aca2.  This
just allows the basics to work for utility queries using Const nodes.

Reviewed-by: Bertrand Drouvot
Discussion: https://postgr.es/m/Y+MRdEq9W9XVa2AB@paquier.xyz