[clang-tidy] Make readability-container-data-pointer use <string> header

[mikecrowe]

Summary

This requires operator[] to be added to the std::basic_string
implementation.

Depends on D144216

[carlosgalvezp]

LGTM, would you mind rebasing to get the pre-merge jobs green?

[mikecrowe]

Rebase on top of fb7ef637a84652dbd3d973a1ba7db9470181b5aa (which is a
descendent of ae25e2f19decb94198301f0726ee613f945cc405 aka D144216, on
which this change relies.)

clang-tools-extra/test/clang-tidy/checkers/Inputs/Headers/string

   _Type& insert(size_type pos, const C* s);
   _Type& insert(size_type pos, const C* s, size_type n);
 
+  _Type& operator[](size_type);
+  const _Type& operator[](size_type) const;
+
   _Type& operator+=(const _Type& str);
   _Type& operator+=(const C* s);
   _Type& operator=(const _Type& str);

clang-tools-extra/test/clang-tidy/checkers/readability/container-data-pointer.cpp

-// RUN: %check_clang_tidy %s readability-container-data-pointer %t -- -- -fno-delayed-template-parsing
+// RUN: %check_clang_tidy %s readability-container-data-pointer %t -- -- -isystem %clang_tidy_headers -fno-delayed-template-parsing
+#include <string>
 
 typedef __SIZE_TYPE__ size_t;
 

   const T &operator[](size_type) const;
 };
 
-template <typename T>
-struct basic_string {
-  using size_type = size_t;
-
-  basic_string();
-
-  T *data();
-  const T *data() const;
-
-  T &operator[](size_t);
-  const T &operator[](size_type) const;
-};
-
-typedef basic_string<char> string;
-typedef basic_string<wchar_t> wstring;
-
 template <typename T>
 struct is_integral;