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.
Should the replacements be MutableArrayRef rather than ArrayRef?