release-20.2: opt: build all partial index predicate expressions in TableMeta

[mgartner]

Backport 2/2 commits from #57707. The backport #58272 relies on these changes.

/cc @cockroachdb/release

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opt: build all partial index predicate expressions in TableMeta

Previously, partial index predicates were only built and added to
TableMeta.PartialIndexPredicates if the scope of the scan in
Builder.buildScan included all ordinary table columns. This behavior
prevented errors when building partial index predicate expressions for
foriegn key check scans. These scans do not include all columns, so when
building a predicate expression that referenced a table column not
included, optbuilder would err with "column does not exist".

This commit changes this behavior so that partial index predicates are
always built and added to TableMeta.PartialIndexPredicates. In order
to do this, Builder.addPartialIndexPredicatesForTable creates its own
table scope. It also constructs a new scan rather than relying on the
already-built scan. Constructing a scan is required for considering
logical properties while normalizing partial index predicates.

See discussion at #57622
for more context on this change.

Release note: None

optbuilder: reduce redundant building of arbiter filter expressions

Previously, the arbiter predicate provided in the WHERE clause of an
INSERT ... ON CONFLICT statement was rebuilt as a memo.FiltersExpr
repetitively while the optbuilder determined arbiter indexes. Now, the
expression is built only once, reducing work.

Release note: None

[cockroach-teamcity]

This change is 

[RaduBerinde]

Reviewable status: complete! 1 of 0 LGTMs obtained (waiting on @RaduBerinde)