pgsql: Disallow COPY FREEZE on partitioned tables

Disallow COPY FREEZE on partitioned tables

This didn't actually work: COPY would fail to flush the right files, and
instead would try to flush a non-existing file, causing the whole
transaction to fail.

Cope by raising an error as soon as the command is sent instead, to
avoid a nasty later surprise. Of course, it would be much better to
make it work, but we don't have a patch for that yet, and we don't know
if we'll want to backpatch one when we do.

Reported-by: Tomas Vondra
Author: David Rowley
Reviewed-by: Amit Langote, Steve Singer, Tomas Vondra

Branch
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REL_11_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/a4db7fe0f71463eb7256d3c42b7a582d3943d604

Modified Files
--------------
doc/src/sgml/perform.sgml | 4 ++--
doc/src/sgml/ref/copy.sgml | 4 +++-
src/backend/commands/copy.c | 29 +++++++++++++++++++++++++++--
src/test/regress/input/copy.source | 29 +++++++++++++++++++++++++++++
src/test/regress/output/copy.source | 23 +++++++++++++++++++++++
5 files changed, 84 insertions(+), 5 deletions(-)