Check for interrupts and stack overflow in TParserGet().

Check for interrupts and stack overflow in TParserGet().

TParserGet() recurses for some token types, meaning it's possible
to drive it to stack overflow.  Since this is a minority behavior,
I chose to add the check_stack_depth() call to the two places that
recurse rather than doing it during every single call.

While at it, add CHECK_FOR_INTERRUPTS(), because this can run
unpleasantly long for long inputs.

Per bug #17995 from Zuming Jiang.  This is old, so back-patch
to all supported branches.

Discussion: https://postgr.es/m/17995-9f20ff3e6389db4c@postgresql.org