Introduce the 'makeArrayRef(...)' family of functions, which fills a similar...

Introduce the 'makeArrayRef(...)' family of functions, which fills a similar role for ArrayRef<> as std::make_pair() fills for std::pair<>: they return the right instantiation of ArrayRef<T> based on the types of the parameters.

They mostly mirror the ArrayRef constructors, with two exceptions:
 * There's no function mirroring the default constructor because it wouldn't have any parameters to deduce the right ArrayRef<T> from.
 * There's an explicit SmallVector<T> overload in addition to the SmallVectorImpl<T> overload. Without it, the single-element overload would try to create an ArrayRef<Smallvector<T> > because it's a better match according to the overloading rules. (And both overloads are used in the current tree, so neither is redundant)

llvm-svn: 135389