[clang-tidy] Add more detection rules for redundant c_str calls.

Summary:
The string class contains methods which support receiving either a string literal or a string object.

For example, calls to append can receive either a char* or a string.
```
  string& append (const string& str);
  string& append (const char* s);
```

Which make these cases equivalent, and the .c_str() useless:
```
  std::string s = "123";
  str.append(s);
  str.append(s.c_str());
```

In these cases, removing .c_str()  doesn't provide any size or speed improvement.
It's only a readability issue.

If the string contains embedded NUL characters,  the string literal and the string
object won't produce the same semantic.

Reviewers: alexfh, sbenza

Subscribers: LegalizeAdulthood, aaron.ballman, chapuni, Eugene.Zelenko, cfe-commits

Differential Revision: http://reviews.llvm.org/D18475

llvm-svn: 266463