pgsql: Reject MERGE in CTEs and COPY

Reject MERGE in CTEs and COPY

The grammar added for MERGE inadvertently made it accepted syntax in
places that were not prepared to deal with it -- namely COPY and inside
CTEs, but invoking these things with MERGE currently causes assertion
failures or weird misbehavior in non-assertion builds.  Protect those
places by checking for it explicitly until somebody decides to implement
it.

Reported-by: Alexey Borzov <borz_off@cs.msu.su>
Discussion: https://postgr.es/m/17579-82482cd7b267b862@postgresql.org

Branch
------
REL_15_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/455d254d22665eb7af3b714435f3fecc63696cc7

Modified Files
--------------
src/backend/commands/copy.c         |  6 ++++++
src/backend/parser/parse_cte.c      |  7 +++++++
src/test/regress/expected/merge.out | 14 ++++++++++++++
src/test/regress/sql/merge.sql      | 10 ++++++++++
4 files changed, 37 insertions(+)