[YAML] Support serializing MutableArrayRef

While the YAML format itself doesn't support fixed-sized arrays, some
of the data structures we encode in and out of YAML (specifically in
ObjectYAML) are actually fixed-sized arrays which we end up expressing
as resizable arrays.

Enabling the YAML tooling to support reading and writing from
fixed-sized arrays using MutableArrayRef can simplify some of the error
reporting and use logic for cases where the sizes of arrays are defined
by the target format.

Note: my SFINAE-foo isn't the best, so if there is a cleaner way to
implement the traits please advise.

Reviewed By: MaskRay

Differential Revision: https://reviews.llvm.org/D140427