swift234

SILGen: Balance the retain counts of guaranteed reabstraction and witness thunk params.

For now, conservatively retain them so they can be forwarded as +1 values, until we can push +0 rvalue support down into the forwarding and function application implementations. Preserve the old, technically incorrect behavior unless -enable-guaranteed-self is enabled to avoid punishing -Onone code until guaranteed self is enabled. (With guaranteed self and +0 rvalue support, the overhead this introduces should be avoidable in the code generator.)

Swift SVN r24374