Support domains over composite types.

This is the last major omission in our domains feature: you can now
make a domain over anything that's not a pseudotype.

The major complication from an implementation standpoint is that places
that might be creating tuples of a domain type now need to be prepared
to apply domain_check().  It seems better that unprepared code fail
with an error like "<type> is not composite" than that it silently fail
to apply domain constraints.  Therefore, relevant infrastructure like
get_func_result_type() and lookup_rowtype_tupdesc() has been adjusted
to treat domain-over-composite as a distinct case that unprepared code
won't recognize, rather than just transparently treating it the same
as plain composite.  This isn't a 100% solution to the possibility of
overlooked domain checks, but it catches most places.

In passing, improve typcache.c's support for domains (it can now cache
the identity of a domain's base type), and rewrite the argument handling
logic in jsonfuncs.c's populate_record[set]_worker to reduce duplicative
per-call lookups.

I believe this is code-complete so far as the core and contrib code go.
The PLs need varying amounts of work, which will be tackled in followup
patches.

Discussion: https://postgr.es/m/4206.1499798337@sss.pgh.pa.us

src/backend/executor/execSRF.c

@@ -502,7 +502,7 @@ restart:
 	{
 		TupleTableSlot *slot = fcache->funcResultSlot;
 		MemoryContext oldContext;
-		bool foundTup;
+		bool		foundTup;
 
 		/*
 		 * Have to make sure tuple in slot lives long enough, otherwise
@@ -734,7 +734,8 @@ init_sexpr(Oid foid, Oid input_collation, Expr *node,
 		/* Must save tupdesc in sexpr's context */
 		oldcontext = MemoryContextSwitchTo(sexprCxt);
 
-		if (functypclass == TYPEFUNC_COMPOSITE)
+		if (functypclass == TYPEFUNC_COMPOSITE ||
+			functypclass == TYPEFUNC_COMPOSITE_DOMAIN)
 		{
 			/* Composite data type, e.g. a table's row type */
 			Assert(tupdesc);