pgsql: Remove dependence on -fwrapv semantics in a few places.

Remove dependence on -fwrapv semantics in a few places.

This commit attempts to update a few places, such as the money,
numeric, and timestamp types, to no longer rely on signed integer
wrapping for correctness.  This is intended to move us closer
towards removing -fwrapv, which may enable some compiler
optimizations.  However, there is presently no plan to actually
remove that compiler option in the near future.

Besides using some of the existing overflow-aware routines in
int.h, this commit introduces and makes use of some new ones.
Specifically, it adds functions that accept a signed integer and
return its absolute value as an unsigned integer with the same
width (e.g., pg_abs_s64()).  It also adds functions that accept an
unsigned integer, store the result of negating that integer in a
signed integer with the same width, and return whether the negation
overflowed (e.g., pg_neg_u64_overflow()).

Finally, this commit adds a couple of tests for timestamps near
POSTGRES_EPOCH_JDATE.

Author: Joseph Koshakow
Reviewed-by: Tom Lane, Heikki Linnakangas, Jian He
Discussion: https://postgr.es/m/CAAvxfHdBPOyEGS7s%2Bxf4iaW0-cgiq25jpYdWBqQqvLtLe_t6tw%40mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/9e9a2b7031f64e49fcaf28f21a4e70eb1212165f

Modified Files
--------------
src/backend/utils/adt/cash.c               |  12 +--
src/backend/utils/adt/numeric.c            |   4 +-
src/backend/utils/adt/numutils.c           |  34 ++++----
src/backend/utils/adt/timestamp.c          |  28 +-----
src/include/common/int.h                   | 131 +++++++++++++++++++++++++++--
src/interfaces/ecpg/pgtypeslib/timestamp.c |  11 +--
src/test/regress/expected/timestamp.out    |  13 +++
src/test/regress/expected/timestamptz.out  |  13 +++
src/test/regress/sql/timestamp.sql         |   4 +
src/test/regress/sql/timestamptz.sql       |   4 +
10 files changed, 189 insertions(+), 65 deletions(-)