[llvm][ADT] Add deduction guides for `MutableArrayRef`

Similar to https://reviews.llvm.org/D140896, this adds deduction guides for the
counterpart of `ArrayRef`: `MutableArrayRef`.  The update plan is the following:

1) Add deduction guides for `MutableArrayRef`.
2) Change uses in-tree from `makeMutableArrayRef` to use deduction guides
3) Mark `makeMutableArrayRef` as deprecated for some time before removing to
   give downstream users time to update.

The deduction guides are similar to those provided by the `makeMutableArrayRef`
function templates, except we don't need one explicitly from `MutableArrayRef`.

Differential Revision: https://reviews.llvm.org/D141327