[clang-tidy] Add a checker that warns on const string & members.

Summary:
Those are considered unsafe and should be replaced with simple pointers or
full copies. It recognizes both std::string and ::string.

Reviewers: alexfh, djasper

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D4522

git-svn-id: https://llvm.org/svn/llvm-project/clang-tools-extra/trunk@213133 91177308-0d34-0410-b5e6-96231b3b80d8