pgsql: Assign collations in partition bound expressions.

Assign collations in partition bound expressions.

Failure to do this can result in errors during evaluation of
the bound expression, as illustrated by the new regression test.

Back-patch to v12 where the ability for partition bounds to be
expressions was added.

Discussion: /messages/by-id/CAJV4CdrZ5mKuaEsRSbLf2URQ3h6iMtKD=hik8MaF5WwdmC9uZw@mail.gmail.com

Branch
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REL_13_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/61a78c71a656593bf4121e624348a990ba5b91da

Modified Files
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src/backend/parser/parse_utilcmd.c | 3 +++
src/test/regress/expected/create_table.out | 7 +++++++
src/test/regress/sql/create_table.sql | 8 ++++++++
3 files changed, 18 insertions(+)