pgsql: Add -Wimplicit-fallthrough to CFLAGS and CXXFLAGS

Add -Wimplicit-fallthrough to CFLAGS and CXXFLAGS

Use it at level 4, a bit more restrictive than the default level, and
tweak our commanding comments to FALLTHROUGH.

(However, leave zic.c alone, since it's external code; to avoid the
warnings that would appear there, change CFLAGS for that file in the
Makefile.)

Author: Julien Rouhaud <rjuju123(at)gmail(dot)com>
Author: Álvaro Herrera <alvherre(at)alvh(dot)no-ip(dot)org>
Reviewed-by: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Discussion: https://postgr.es/m/20200412081825.qyo5vwwco3fv4gdo@nol
Discussion: https://postgr.es/m/flat/E1fDenm-0000C8-IJ(at)gemulon(dot)postgresql(dot)org

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/3e9744465dbe51822c7d76baca1f934d54ba9452

Modified Files
--------------
configure | 91 +++++++++++++++
configure.in | 2 +
src/backend/access/heap/heapam_handler.c | 2 +-
src/backend/catalog/dependency.c | 2 +-
src/backend/commands/tablecmds.c | 2 +-
src/backend/commands/trigger.c | 2 +-
src/backend/executor/nodeHash.c | 14 +--
src/backend/executor/nodeHashjoin.c | 10 +-
src/backend/executor/nodeLimit.c | 4 +-
src/backend/libpq/auth.c | 2 +-
src/backend/optimizer/util/clauses.c | 4 +-
src/backend/parser/parse_utilcmd.c | 2 +-
src/backend/partitioning/partprune.c | 8 +-
src/backend/postmaster/postmaster.c | 6 +-
src/backend/regex/regc_pg_locale.c | 44 ++++----
src/backend/replication/logical/reorderbuffer.c | 2 +-
src/backend/replication/walreceiver.c | 2 +-
src/backend/replication/walreceiverfuncs.c | 2 +-
src/backend/tcop/utility.c | 2 +-
src/backend/utils/adt/formatting.c | 2 +-
src/backend/utils/adt/jsonb_util.c | 2 +-
src/backend/utils/adt/timestamp.c | 54 ++++-----
src/backend/utils/adt/tsginidx.c | 2 +-
src/backend/utils/hash/dynahash.c | 2 +-
src/backend/utils/mb/mbutils.c | 6 +-
src/backend/utils/misc/guc.c | 2 +-
src/common/hashfn.c | 144 ++++++++++++------------
src/common/wchar.c | 6 +-
src/interfaces/ecpg/pgtypeslib/interval.c | 2 +-
src/interfaces/libpq/fe-secure.c | 2 +-
src/pl/plpgsql/src/pl_exec.c | 6 +-
src/port/snprintf.c | 2 +-
src/timezone/Makefile | 5 +
33 files changed, 269 insertions(+), 171 deletions(-)