53067: opt: add INSERT ON CONFLICT WHERE syntax for selecting arbiter indexes r=mgartner a=mgartner

This commit adds support for selecting unique partial indexes as
"arbiters" for `INSERT ON CONFLICT` statements. An arbiter index is a
unique index that is used to detect conflicts in which to perform the
`ON CONFLICT` action on.

Unique partial indexes can only be used as arbiters if the arbiter
predicate in the `ON CONFLICT` statement implies the partial index's
predicate.

To avoid parsing the same partial index predicates multiple times, this
commit also adds a `parsedIndexExprs` cache to `mutationBuilder`,
similar to the cache it has for default and computed column expressions.

Finally, this commit also makes the parsing of `ON CONFLICT DO UPDATE`
more strict. It now requires a list of conflict columns. This matches
Postgres behavior.

From https://www.postgresql.org/docs/12/sql-insert.html:

> For ON CONFLICT DO UPDATE, a conflict_target must be provided.

In Postgres, the `conflict_target` can be a list of columns or a
constraint name. Cockroach currently only supports a list of column
names as a conflict target, so the list of column names preceeding
`DO UPDATE` is now required by the parser.

Release note (sql change): An `INSERT ... ON CONFLICT DO UPDATE`
statement without a list of column names after `ON CONFLICT` now results
in a SQL syntax error with the error code 42601. Previously it errored
with the message "there is no unqiue or exlcusion constraint matching
the ON CONFLICT specification" and the error code 42P10.


53126: sql: drop schemas when dropping parent database r=rohany a=rohany

Fixes #52362.

This commit updates `DROP DATABASE CASCADE` to also clean up any child
user defined schemas.

Release note: None

Co-authored-by: Marcus Gartner <marcus@cockroachlabs.com>
Co-authored-by: Rohan Yadav <rohany@alumni.cmu.edu>