[clang-tidy] Provide default template arguments in <string>

[mikecrowe]

Summary

Simplify the use of the basic_string and basic_string_view types by
providing default template arguments.

Depends on D145311

[PiotrZSL]

LGTM

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

 #ifndef _STRING_
 #define _STRING_
 
 // For size_t
 #include <string.h>
 
 typedef unsigned __INT16_TYPE__ char16;
 typedef unsigned __INT32_TYPE__ char32;
 
 namespace std {
 template <typename T>
 class allocator {};
 template <typename T>
 class char_traits {};
-template <typename C, typename T, typename A>
+template <typename C, typename T = char_traits<C>, typename A = allocator<C>>
 struct basic_string {
   typedef size_t size_type;
   typedef basic_string<C, T, A> _Type;
   basic_string();
   basic_string(const C *p, const A &a = A());
   basic_string(const C *p, size_type count);
 
   ~basic_string();
struct basic_string {
   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;
-typedef basic_string<char16, std::char_traits<char16>, std::allocator<char16>> u16string;
-typedef basic_string<char32, std::char_traits<char32>, std::allocator<char32>> u32string;
+typedef basic_string<char> string;
+typedef basic_string<wchar_t> wstring;
+typedef basic_string<char16> u16string;
+typedef basic_string<char32> u32string;
 
-template <typename C, typename T>
+template <typename C, typename T = char_traits<C>>
 struct basic_string_view {
   basic_string_view(const C* s);
 };
-typedef basic_string_view<char, std::char_traits<char>> string_view;
-typedef basic_string_view<wchar_t, std::char_traits<wchar_t>> wstring_view;
-typedef basic_string_view<char16, std::char_traits<char16>> u16string_view;
-typedef basic_string_view<char32, std::char_traits<char32>> u32string_view;
+typedef basic_string_view<char> string_view;
+typedef basic_string_view<wchar_t> wstring_view;
+typedef basic_string_view<char16> u16string_view;
+typedef basic_string_view<char32> u32string_view;
 
 std::string operator+(const std::string&, const std::string&);
 std::string operator+(const std::string&, const char*);
 std::string operator+(const char*, const std::string&);
 
 bool operator==(const std::string&, const std::string&);
 bool operator==(const std::string&, const char*);
 bool operator==(const char*, const std::string&);