Teach get_expr_result_type() to manufacture a tuple descriptor directly
from a RowExpr node.  If the RowExpr has type RECORD, this is the only
way to get a tupdesc for its result, since even if the rowtype has been
blessed, we don't have its typmod available at this point.  (If the
RowExpr has some named composite type, we continue to let the existing
code handle it, since the RowExpr might well not have the correct column
names embedded in it.)

This fixes assorted corner cases illustrated by the added regression
tests.

Discussion: https://postgr.es/m/10872.1572202006@sss.pgh.pa.us