Add a planner support function for starts_with().

This fills in some gaps in planner support for starts_with() and
the equivalent ^@ operator:

* A condition such as "textcol ^@ constant" can now use a regular
btree index, not only an SP-GiST index, so long as the index's
collation is C.  (This works just like "textcol LIKE 'foo%'".)

* "starts_with(textcol, constant)" can be optimized the same as
"textcol ^@ constant".

* Fixed-prefix LIKE and regex patterns are now more like starts_with()
in another way: if you apply one to an SPGiST-indexed column, you'll
get an index condition using ^@ rather than two index conditions with
>= and <.

Per a complaint from Shay Rojansky.  Patch by me; thanks to
Nathan Bossart for review.

Discussion: https://postgr.es/m/232599.1633800229@sss.pgh.pa.us

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