pgsql: Dial back -Wimplicit-fallthrough to level 3

Dial back -Wimplicit-fallthrough to level 3

The additional pain from level 4 is excessive for the gain.

Also revert all the source annotation changes to their original
wordings, to avoid back-patching pain.

Discussion: https://postgr.es/m/31166.1589378554@sss.pgh.pa.us

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/17cc133f017cb13737e23ce0da4415daf2c34cc3

Modified Files
--------------
configure | 40 +++----
configure.in | 4 +-
contrib/pgcrypto/pgp-info.c | 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 -
34 files changed, 194 insertions(+), 199 deletions(-)