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: https://postgr.es/m/CAJV4CdrZ5mKuaEsRSbLf2URQ3h6iMtKD=hik8MaF5WwdmC9uZw@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/72647ac3bf0f11732483eac2cd9b5cf4972f2e1f

Modified Files
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src/backend/parser/parse_utilcmd.c         | 1 +
src/test/regress/expected/create_table.out | 7 +++++++
src/test/regress/sql/create_table.sql      | 8 ++++++++
3 files changed, 16 insertions(+)