Add makeArrayRef() overload for ArrayRef input (no-op/identity) NFC

The purpose is to allow templated wrapper to work with either
ArrayRef or any convertible operation:

template<typename Container>
void wrapper(const Container &Arr) {
  impl(makeArrayRef(Arr));
}

with Container being a std::vector, a SmallVector, or an ArrayRef.

From: Mehdi Amini <mehdi.amini@apple.com>
llvm-svn: 247214