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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REL_12_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/29f20db85ed220f075fe8181bac49269611f13cb

Modified Files
--------------
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(+)