pgsql: Fix additional fallthrough warnings from clang

Fix additional fallthrough warnings from clang

Clang warns if falling through to a case or default label that is
immediately followed by break, but GCC does
not (https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91432) (MSVC also
warns about the equivalent code in C++.)

This is in preparation for enabling fallthrough warnings on Clang.

Reviewed-by: Jelte Fennema-Nio <postgres(at)jeltef(dot)nl>
Discussion: https://www.postgresql.org/message-id/flat/76a8efcd-925a-4eaf-bdd1-d972cd1a32ff%40eisentraut.org

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/3a63b76571ba253e2784ba34377cdcbdb8ad6796

Modified Files
--------------
contrib/isn/isn.c | 1 +
contrib/pg_overexplain/pg_overexplain.c | 1 +
src/backend/access/transam/xlogrecovery.c | 1 +
src/backend/replication/logical/decode.c | 3 +--
src/backend/tcop/dest.c | 1 +
src/backend/utils/mb/conversion_procs/euc_tw_and_big5/big5.c | 1 +
src/pl/plpgsql/src/pl_comp.c | 1 +
src/tools/pg_bsd_indent/indent.c | 1 +
8 files changed, 8 insertions(+), 2 deletions(-)