Avoid using rvalue references with trivially copyable types.

Summary:
When the dereference operator returns a value that is trivially
copyable (like a pointer), copy it. After this change, modernize-loop-convert
check can be applied to the whole llvm source code without breaking any build
or test.

Reviewers: alexfh, klimek

Subscribers: alexfh, cfe-commits

Differential Revision: http://reviews.llvm.org/D12675

llvm-svn: 246989