When a `borrowing` or `consuming` parameter is captured by a closure,
we emit references to the binding within the closure as if it is non-implicitly
copyable, but we didn't mark the bindings inside the closure for move-only
checking to ensure the uses were correct, so improper consumes would go
undiagnosed and lead to assertion failures, compiler crashes, and/or
miscompiles. Fixes rdar://127382105
It works well enough now that it should be an acceptable replacement for both
borrowing and consuming switches that works in more correct situations than the
previous implementation. This does however expose a few known issues that I'll
try to fix in follow ups:
- overconsumes cause verifier errors instead of raising diagnostics (rdar://125381446)
- cases with multiple pattern labels aren't yet supported (rdar://125188955)
- copyable types with the `borrowing` or `consuming` modifiers should probably use
noncopyable pattern matching.
The `BorrowingSwitch` flag is still necessary to enable the surface-level syntax
changes (switches without `consume` and the `_borrowing` modifier, for instance).
