[clang-tidy] New checker to detect suspicious string constructor.

Summary:
Checker to validate string constructor parameters.

A common mistake is to swap parameter for the fill-constructor.
```
  std::string str('x', 4);
  std::string str('4', x);
```

Reviewers: alexfh

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D19146

llvm-svn: 267011