src: constrain MaybeStackBuffer::ToString and ToStringView to standard char types

On newer libc++ (shipped with macOS Xcode 16+), std::char_traits<T> for
T not equal to char, wchar_t, char8_t, char16_t, or char32_t is
deprecated and will be removed in a future release. When
MaybeStackBuffer is instantiated with unsigned char or uint8_t (e.g. in
test/cctest/test_util.cc), the ToString() and ToStringView() methods
trigger this deprecation warning because they reference
std::basic_string<unsigned char> and std::basic_string_view<unsigned
char>, even though these methods are never actually called for those
types.

Add C++20 requires clauses to constrain ToString() and ToStringView()
to only be available for standard character types, preventing the
deprecated std::char_traits<unsigned char> from being instantiated.

This is consistent with the existing use of C++20 concepts elsewhere
in the same file (e.g. the is_callable concept).

Author: om-ghante

src/util.h

@@ -503,8 +503,18 @@ class MaybeStackBuffer {
       free(buf_);
   }
 
-  inline std::basic_string<T> ToString() const { return {out(), length()}; }
-  inline std::basic_string_view<T> ToStringView() const {
+  inline std::basic_string<T> ToString() const
+    requires(std::is_same_v<T, char> || std::is_same_v<T, wchar_t> ||
+             std::is_same_v<T, char8_t> || std::is_same_v<T, char16_t> ||
+             std::is_same_v<T, char32_t>)
+  {
+    return {out(), length()};
+  }
+  inline std::basic_string_view<T> ToStringView() const
+    requires(std::is_same_v<T, char> || std::is_same_v<T, wchar_t> ||
+             std::is_same_v<T, char8_t> || std::is_same_v<T, char16_t> ||
+             std::is_same_v<T, char32_t>)
+  {
     return {out(), length()};
   }
   // This can only be used if the buffer contains path data in UTF8