swift234

SIL/Projection: support for index_addr.

Moved the lightweight access path helper into a ProjectionIndex utility.
Added support to it and the kitchen-sink Projection utility.

Add a ProjectionIndex utility that supports IndexAddr and does only what it needs to do.

While I'm at it, add IndexAddr support to Projection and
ProjectionPath so that other passes can make use of it.
Unfortunately, LoadStore optimization is still not strong enough to
benefit, so I don't have a test case yet.

Note that we have to be very careful now not to assume that when one
projection path is a subsequence of another it must address an
containing region of memory. The implicit index_addr #0 is not part of
the projection path!

Swift SVN r29167