When visiting consumes, also visit `extend_lifetime` instructions.
These instructions are not lifetime ending, but together with the
consumes, they enclose the users of a value.
Add a flag to LinearLiveness to control whether these instructions are
added so that the verifier can use verify that all such instructions
appear outside the linear lifetime boundary (not including them).
"reborrow" flag on the SILArgument avoids transitive walk over the phi operandsi
to determine if it is a reborrow in multiple utilities.
SIL transforms must keep the flag up-to-date by calling SILArgument::setReborrow.
SILVerifier checks to ensure the flag is not invalidated.
Currently "escaping" is not used anywhere.
Since partial_apply [on_stack] is considered as a BorrowingOperand now,
and BorrowingOperand::visitScopeEndingUses can be called on the its forwarded values
lifetime ends, update the assertion to reflect this.
Encapsulate all the complexity of reborrows and guaranteed phi in 3
ownership liveness interfaces:
LinerLiveness, InteriorLiveness, and ExtendedLiveness.
