swift234

[deserialization] Begin move to lazy linking by putting a SerializedModuleLoader on SILModule.

Right now it is not wired up. That is coming in forthcoming commits. I am trying
to be very incremental about this.

This series of patches will fix an assertion triggered by us not deserializing
functions from vtables appropriately. Doing linking lazily fixes that issue and
does the "right thing".

The work is being done in consultation with John, Joe, and Jordan.

Swift SVN r14479