crash with sql language partition support function

Hi.

I noticed that RelationBuildPartitionKey() is not doing the right thing
with respect to which context it passes to fmgr.c to set a the (cached)
partition support function's FmgrInfo's fn_mcxt.

I noticed a crash while trying to change partition pruning tests to use
manually created hash operator class per [1].

CREATE OR REPLACE FUNCTION hashint4_noop(int4, int8) RETURNS int8 AS
$$SELECT coalesce($1)::int8$$ LANGUAGE sql IMMUTABLE STRICT;

CREATE OPERATOR CLASS test_int4_ops FOR TYPE int4 USING HASH AS
OPERATOR 1 = , FUNCTION 2 hashint4_noop(int4, int8);

CREATE OR REPLACE FUNCTION hashtext_length(text, int8) RETURNS int8 AS
$$SELECT length(coalesce($1))::int8$$ LANGUAGE sql IMMUTABLE STRICT;

CREATE OPERATOR CLASS test_text_ops FOR TYPE text USING HASH AS
OPERATOR 1 = , FUNCTION 2 hashtext_length(text, int8);

create table hp (a int, b text) partition by hash (a test_int4_ops, b
test_text_ops);
create table hp0 partition of hp for values with (modulus 4, remainder 0);
create table hp3 partition of hp for values with (modulus 4, remainder 3);
create table hp1 partition of hp for values with (modulus 4, remainder 1);
create table hp2 partition of hp for values with (modulus 4, remainder 2);

insert into hp values (1, 'abcde');
INSERT 0 1
Time: 13.604 ms
postgres=# insert into hp values (null, 'abcde');
server closed the connection unexpectedly
    This probably means the server terminated abnormally
    before or while processing the request.
The connection to the server was lost. Attempting reset: Failed.

Attached fixes it.  It teaches RelationBuildPartitionKey() to use
fmgr_info_cxt and pass rd_partkeycxt to it.

Since this bug also exists in the released PG 10 branch, I also created a
patch for that.  It's slightly different than the one for PG 11dev,
because there were some changes recently to how the memory context is
manipulated in RelationBuildPartitionKey.  That's
v1-PG10-0001-Fix-a-memory-context-bug-in-RelationBuildPartitio.patch.

Thanks,
Amit

[1]
https://www.postgresql.org/message-id/CA%2BTgmoZ0D5kJbt8eKXtvVdvTcGGWn6ehWCRSZbWytD-uzH92mQ%40mail.gmail.com