Remove dependence on integer wrapping

Hi,

In [0] Andres suggested enabling -ftrapv in assert enabled builds. While
I vastly underestimated the complexity of updating `configure` to do
this, I was able to enable the flag locally. Enabling this flag causes
our existing regression tests to trap and fail in multiple different
spots. The attached patch resolves all of these overflows so that all
of our existing tests will pass with the -ftrapv flag enabled.

Some notes on the patch itself are:

I originally added the helper functions to int.h thinking I'd find
many more instances of overflow due to integer negation, however I
didn't find that many. So let me know if you think we'd be better
off without the functions.

I considered using #ifdef to rely on wrapping when -fwrapv was
enabled. This would save us some unnecessary branching when we could
rely on wrapping behavior, but it would mean that we could only enable
-ftrapv when -fwrapv was disabled, greatly reducing its utility.

The following comment was in the code for parsing timestamps:

/* check for just-barely overflow (okay except time-of-day wraps) */
/* caution: we want to allow 1999-12-31 24:00:00 */

I wasn't able to fully understand it even after staring at it for
a while. Is the comment suggesting that it is ok for the months field,
for example, to wrap around? That doesn't sound right to me I tested
the supplied timestamp, 1999-12-31 24:00:00, and it behaves the same
before and after the patch.

Thanks,
Joe Koshakow

[0]
https://www.postgresql.org/message-id/20240213191401.jjhsic7et4tiahjs%40awork3.anarazel.de