[SIL.rst] Add docs explaining how Move Only Types can be copied in Raw SIL, but not in later SIL stages.
[SIL.rst] Add docs explaining how Move Only Types can be copied in Raw SIL, but not in later SIL stages.

Since I am going to start enforcing this invariant more broadly, I want to
document it in SIL.rst. I also included a discussion explaining why we maintain
this invariant: it enables SILGen to use a simple algorithm to insert copies/us
to clean that up using a diagnostic guaranteed optimization pass which runs on
Raw SIL.

glessard/swift,glessard/swift,benlangmuir/swift,apple/swift,roambotics/swift,benlangmuir/swift,roambotics/swift,apple/swift,apple/swift,benlangmuir/swift,glessard/swift,roambotics/swift,roambotics/swift,apple/swift,benlangmuir/swift,benlangmuir/swift,glessard/swift,roambotics/swift,apple/swift,glessard/swift,benlangmuir/swift,roambotics/swift,glessard/swift,apple/swift