Selectively include window frames in expression walks/mutates.

query_tree_walker and query_tree_mutator were skipping the
windowClause of the query, without regard for the fact that the
startOffset and endOffset in a WindowClause node are expression trees
that need to be processed. This was an oversight in commit ec4be2ee
from 2010 which added the expression fields; the main symptom is that
function parameters in window frame clauses don't work in inlined
functions.

Fix (as conservatively as possible since this needs to not break
existing out-of-tree callers) and add tests.

Backpatch all the way, since this has been broken since 9.0.

Per report from Alastair McKinley; fix by me with kibitzing and review
from Tom Lane.

Discussion: https://postgr.es/m/DB6PR0202MB2904E7FDDA9D81504D1E8C68E3800@DB6PR0202MB2904.eurprd02.prod.outlook.com