[clang-tidy] Add support for different char-types for the...

[clang-tidy] Add support for different char-types for the readability-redundant-string-cstr checker.

Summary:
The current checker is able to recognize std::string but does not recognize other string variants.
This patch is adding the support for any string defined with basic_string without considering the
the underlying char type.

The most common variant is: 'std::wstring' based on 'wchar_t'.

There are also other string variants added to the standard: u16string, u32string, etc...

Reviewers: alexfh

Subscribers: mamai, dblaikie, cfe-commits

Differential Revision: http://reviews.llvm.org/D18412

llvm-svn: 264325