[clang-tidy] Fix redundant-string-init check with msvc 14 headers.

Summary:
The string constructors are not defined using optional parameters and are not recognized by the redundant-string-init checker.

The following patch fixes the redundant-string-init checker for the Visual Studio 14 headers file.
The matcher now accept both variant (with 1 and 2 parameters).

Also added new unittests.

Similar issue than: [[ http://reviews.llvm.org/D18285 | review ]]

In the xstring.h header, the constructors are defined this way:
```
basic_string(const _Myt& _Right) [...]
basic_string(const _Myt& _Right, const _Alloc& _Al) [...]
```

Reviewers: alexfh, hokein

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D18293

llvm-svn: 264069