Move from llvm::makeArrayRef to ArrayRef deduction guides

Since we're now requiring C++17, Let's get rid of makeXXX functions like
makeArrayRef, and use deduction guides instead.

This is a first step: Introduce the deduction guide. Following steps
will be a) use them and b) deprecate makeArrayRef.

Apart from codebase modernization, there isn't much benefit from that
move, but I can still mention that it would slightly (probably
negligibly) decrease the number of symbols / debug info, as deduction
guides don't generate new code.

Differential Revision: https://reviews.llvm.org/D140896