Add ADL support to range based <algorithm> extensions

This adds support for ADL in the range based <algorithm> extensions
(llvm::for_each etc.).

Also adds the helper functions llvm::adl::begin and llvm::adl::end which wrap
std::begin and std::end with ADL support.

Saw this was missing from a recent llvm weekly post about adding llvm::for_each
and thought I might add it.

Patch by Stephen Dollberg!

Differential Revision: https://reviews.llvm.org/D40006

llvm-svn: 318703