[rbi] Implement support for non-Sendable base values.

There are a few major changes here:

1. We now return a TrackableValue from getTrackableValue() if we have either a
non-Sendable value or a non-Sendable base. This means that we /will/ return
TrackableValues that may have a Sendable value or a Sendable base. To make it
easier to work with this, I moved the isSendable check and the do I have a base
check into PartitionOpBuilder. So, most of the actual code around emitting
values does not need to reason about this. They can just call addRequire or
addSend and pass in either TrackableValue::value or TrackableValue::base without
needing to check if the former is non-Sendable or if the latter is non-Sendable
and non-nil.

2. I searched all of the places where we were grabbing trackable values and
inserted require checks for the base value as appropriate.

Both of these together have prevented the code from becoming too heavy.

This fixes https://forums.swift.org/t/lets-debug-missing-rbi-data-race-diagnostics/78910

rdar://149019222
(cherry picked from commit 6d8b9b0)

Author: gottesmm