Arrange for an explicit cast applied to an ARRAY[] constructor to be applied

directly to all the member expressions, instead of the previous implementation
where the ARRAY[] constructor would infer a common element type and then we'd
coerce the finished array after the fact.  This has a number of benefits,
one being that we can allow an empty ARRAY[] construct so long as its
element type is specified by such a cast.

Brendan Jurd, minor fixes by me.

src/test/regress/sql/arrays.sql

@@ -280,6 +280,7 @@ select E'{{1,2},\\{2,3}}'::text[];
 select '{{"1 2" x},{3}}'::text[];
 select '{}}'::text[];
 select '{ }}'::text[];
+select array[];
 -- none of the above should be accepted
 
 -- all of the following should be accepted
@@ -292,6 +293,7 @@ select '{
            0 second,
            @ 1 hour @ 42 minutes @ 20 seconds
          }'::interval[];
+select array[]::text[];
 -- all of the above should be accepted
 
 -- tests for array aggregates