[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

struct basic_string {
   size_type find(const _Type& str, size_type pos = 0) const;
   size_type find(const C* s, size_type pos = 0) const;
   size_type find(const C* s, size_type pos, size_type n) const;
 
   _Type& insert(size_type pos, const _Type& str);
   _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);
   _Type& operator=(const C* s);
 };
 
 typedef basic_string<char, std::char_traits<char>, std::allocator<char>> string;
 typedef basic_string<wchar_t, std::char_traits<wchar_t>, std::allocator<wchar_t>> wstring;

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;
 
 namespace std {
 template <typename T>
 struct vector {
   using size_type = size_t;
 
   vector();
   explicit vector(size_type);
 
   T *data();
   const T *data() const;
 
   T &operator[](size_type);
   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;
 
 template <>
 struct is_integral<size_t> {
   static const bool value = true;
 };
 
 template <bool, typename T = void>