Only copy small types in modernize-loop-convert.

Summary: If the size of the type is above a certain bound, we'll take a const reference. This bound can be set as an option. For now, the default value is 16 bytes.

Reviewers: klimek

Subscribers: alexfh, cfe-commits

Differential Revision: http://reviews.llvm.org/D14176

llvm-svn: 251694