Issue Description:

In the standard library's Shims.swift, there are these declarations:

/// A dummy value to be used as the target for `mutationsPtr` in fast
/// enumeration implementations.
internal var _fastEnumerationStorageMutationsTarget: CUnsignedLong = 0

/// A dummy pointer to be used as `mutationsPtr` in fast enumeration
/// implementations.
public // SPI(Foundation)
var _fastEnumerationStorageMutationsPtr: UnsafeMutablePointer<CUnsignedLong> {
  return UnsafeMutablePointer(
      Builtin.addressof(&_fastEnumerationStorageMutationsTarget))
}

There's no reason to expose this publicly. The way NSFastEnumeration works, the mutationsPtr either points to a word-sized chunk of memory that will change if the collection is mutated (to catch mutation-while-iterating bugs), or it points to some constant memory. That means the standard library and the Foundation overlay can each use the address of their own global variable if they don't want to bother checking for mutations, and they don't need a wrapper function to do it.