[llvm][ADT] Mark `makeMutableArrayRef` as deprecated

Now that all of the uses of `makeMutableArrayRef` are replaced in-tree with use
of deduction guides (see
https://github.com/llvm/llvm-project/commit/a288d7f937708cf67d960962bfa22ffae37ddbf4),
mark `makeMutableArrayRef` as deprecated.

Also remove the old tests for `makeMutableArrayRef` in favor of the ones
introduced with the deduction guides in
https://github.com/llvm/llvm-project/commit/38791259c1165cedfa313e06dc20e443f1e20634.

Differential Revision: https://reviews.llvm.org/D141872