silgen: allow borrow of subscript for noncopyables #83959
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If a subscript uses a read accessor to yield a noncopyable value, we'd emit an `end_apply` that's too tightly scoped to allow for a subsequent borrowing access on the yielded value. resolves rdar://159079818
We were not able to use an existential as the base of an access that strictly borrows the existential, because SILGen's RValue emission would establish a fresh evaluation scope just for the existential's opening, and then copy the opened value out. This is problematic for noncopyable existentials. So this patch moves & adds FormalEvaluationScope's around so they're broad enough to enable a borrow of an existential. The idea behind this refactoring is to establish top-level FormalEvaluationScopes when initially creating RValue's for Expr's in SILGen. Any more-tightly scoped operations will already establish their own nested scope, so this is mostly adding safe-guards.
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If a subscript uses a read accessor to yield a noncopyable value, we'd emit an
end_applythat's too tightly scoped to allow for a subsequent borrowing access on the yielded value.resolves rdar://159079818