Fix MutableSpan exclusive access to unsafe pointers

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This fix enables exclusive access to a MutableSpan created from an UnsafeMutablePointer.

The compiler has a special case that allows MutableSpan to depend on a mutable
pointer without extending that pointer's access scope. That lets us implement
standard library code like this:

mutating public func extracting(droppingLast k: Int) -> Self {
  //...
  let newSpan = unsafe Self(_unchecked: _pointer, byteCount: newCount)
  return unsafe _overrideLifetime(newSpan, mutating: &self)

Refine this special case so that is does not apply to inout parameters where the
programmer has an expectation that the unsafe pointer is not copied when being
passed as an argument. Now, we safely get an exclusivity violation when creating
two mutable spans from the same pointer field:

@lifetime(&self)
mutating func getSpan() -> MutableSpan<T> {
  let span1 = makeMutableSpan(&self.pointer)
  let span2 = makeMutableSpan(&self.pointer) // ERROR: overlapping access
  return span1
}

If we don't fix this now, it will likely be source breaking in the future.

Fixes rdar://153745332 (Swift allows constructing two MutableSpans to the same underlying pointer)

[atrick]

@swift-ci test

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@swift-ci test macOS

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@swift-ci test Windows

[meg-gupta]

lgtm