As part of validating new schema elements (unique indexes, constraints)
in the schema changer, we inject in-memory descriptors to be used by the
internal executor that are never written to disk.

This is currently done by creating an entirely new dummy
`descs.Collection` and adding the injected descriptors as uncommitted
descriptors, and then setting this collection as the `tcModifier` on the
internal executor.  Then all subsequent queries using the internal
executor, which each get their own child `connExecutor` and
`descs.Collection`, will have their collection's uncommitted descriptors
replaced by the ones in the dummy collection.

This commit introduces the concept of a "synthetic descriptor" to refer
to these injected descriptors, and slightly improves the interface in
two ways:

1. Instead of creating a new `descs.Collection` to hold synthetic
   descriptors to copy, we now just use a slice of
   `catalog.Descriptor`s.
2. Synthetic descriptors are now made explicit in the `descs.Collection`
   and precede uncommitted descriptors when resolving immutable
   descriptors. Resolving these descriptors mutably is now illegal and
   will return an assertion error.

This commit doesn't change the fact that the synthetic descriptors to be
injected into each query/statement's child `descs.Collection` are set
on the internal executor itself. This is still not threadsafe. It would
make more sense for these descriptors to be scoped at the level of
individual queries.

Release note: None