Add support for 'cbegin()' and 'cend()' on modernize-loop-convert.

Summary:
This fixes https://llvm.org/bugs/show_bug.cgi?id=22196 .

Also add a non-trivially copyable type to fix some tests that were meant to be about using const-refs, but were changed in r248438.

Reviewers: klimek

Subscribers: alexfh, cfe-commits

Differential Revision: http://reviews.llvm.org/D13292

llvm-svn: 248994