swift234

SIL.rst: Clarify UB semantics of project_existential insns.

Avoid the SSA watch word "use". The projected address or class reference from an existential can be spilled, phi-ed, or GVNed, but it's UB if it's passed to a protocol method from another existential, or if the address is dereferenced after the existential has been modified.

Swift SVN r6621