PR #97415 - collatedstring: support default, C, and POSIX in expressions

[cockroach-teamcity]

Exalate commented:

Related PR: cockroachdb/cockroach#97415
Commit: cockroachdb/cockroach@8c1e1be
Fixes: cockroachdb/cockroach#50734 cockroachdb/cockroach#95667
Informs: cockroachdb/cockroach#57255

---

Release note (sql change): Expressions of the form COLLATE "default",
COLLATE "C", and COLLATE "POSIX" are now supported. Since the
default collation cannot be changed currently, these expressions are all
equivalent. The expressions are evaluated by treating the input as a
normal string, and ignoring the collation.

This means that comparisons between strings and collated strings that
use "default", "C", or "POSIX" are now supported.

Creating a column with the "C" or "POSIX" collations is still not
supported.

Jira Issue: DOC-7012