Do parse analysis of an EXPLAIN's contained statement during the normal

Do parse analysis of an EXPLAIN's contained statement during the normal
parse analysis phase, rather than at execution time.  This makes parameter
handling work the same as it does in ordinary plannable queries, and in
particular fixes the incompatibility that Pavel pointed out with plpgsql's
new handling of variable references.  plancache.c gets a little bit
grottier, but the alternatives seem worse.