pgsql: get_expr_result_type has to be prepared to pull type information

Log Message:
-----------
get_expr_result_type has to be prepared to pull type information
from a RECORD Const node, because that's what it may be faced with
after constant-folding of a function returning RECORD. Per example
from Michael Fuhr.

Modified Files:
--------------
pgsql/src/backend/utils/fmgr:
funcapi.c (r1.21 -> r1.22)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/fmgr/funcapi.c.diff?r1=1.21&r2=1.22)