Rewrite the type of `nonisolated(nonsending)` closures.
The constraint solver does not reliably give closures a function type
that includes `nonisolated(noncaller)`, even when the immediate context
requires a conversion to such a type. We were trying to work around this
in SILGen, but the peephole only kicked in if the types matched exactly,
so a contextual conversion that e.g. added `throws` was still emitting
the closure as `@concurrent`, which is of course the wrong semantics.
It's relatively easy to avoid all this by just rewriting the closure's
type to include `nonisolated(nonsending)` at a point where we can reliably
decide that, and then SILGen doesn't have to peephole anything for
correctness.
Fixes rdar://155313349
