[ADT] Add makeVisitor to STLExtras.h

Adds a utility to combine multiple Callables into a single Callable.
This is useful to make constructing a visitor for `std::visit`-like
functions more natural; functions like this will be added in future
patches.

Intended to supercede https://reviews.llvm.org/D99560 by
perfectly-forwarding the combined Callables.

Reviewed By: dblaikie

Differential Revision: https://reviews.llvm.org/D100670