MoveOnlyChecker: avoid dangling reference in custom alloc
The AvailableValueStore maintains stable spans of SILValues,
one span (effectively a MutableArrayRef) per basic block, that
is preallocated for performance.
This patch fixes an issue where _a pointer_ to a span is vended from
`AvailableValueStore::get` and that pointer is into storage within
a DenseMap, which can reallocate and invalidate all such pointers.
There's no reason to be returning a pointer to a MutableArrayRef,
aka AvailableValues, so I've simply stripped off that indirection to
fix this issue.
resolves rdar://162440304
(cherry picked from commit d1f3f8495e5f754fd4694b4afddd510c549cdf85)
