Mostly, move the responsibilities of the check_call() function to the
callers, transformAggregateCall() and transformWindowFuncCall().

This fixes one long-standing, albeit harmless, bug. Previously, you got an
"Unexpected internal error", if you tried to use a window function in the
WHERE clause of a DELETE statement, instead of a user-friendly syntax
error. Add a test case for that.

Move a few similar tests from 'olap_window_seq' to 'qp_olap_windowerr'.
Seems like a more appropriate place for them. Also, 'olap_window_seq' has
an alternative expected output file for ORCA, so it's nice to keep tests
that produce the same output with or without ORCA out of there. Also add a
test query for creating an index on an expression containing a window
function. There was a test for that already, but it was missing parens
around the expression, and therefore produced an error already in the
grammar.