[gmock] Teach gmock ElementsAre and BeginEndDistanceIs matchers to

handle generic ranges by using std::begin and std::end rather than
requiring things to look exactly like an STL container.

Much of the credit for this goes to Dave Blaikie who helped me figure
out the right incantations.

This will probably be re-designed when I send this to the maintainers of
gmock, so I've instead structured it to change is little as possible
while it is a local patch. That makes it somewhat ugly, but I think a focused
change is better for getting this to work for LLVM today and letting the
upstream maintainers figure out the correct long-term pattern.

Differential Revision: https://reviews.llvm.org/D28288

llvm-svn: 291623