[libc++] Make __allocate_long_buffer a constructor of __long

[philnik777]

We currently have __allocate_long_buffer as a static function inside basic_string. This functions basically just creates a __long string representation, so this patch makes it a constructor of that internal struct instead. This reduces the library-internal interface a bit, making it less likely to be misused.

[github-actions]

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git-clang-format --diff origin/main HEAD --extensions  -- libcxx/include/string --diff_from_common_commit

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View the diff from clang-format here.

diff --git a/libcxx/include/string b/libcxx/include/string
index be3c52d8a..bff88dda3 100644
--- a/libcxx/include/string
+++ b/libcxx/include/string
@@ -867,11 +867,11 @@ private:
   // If the MSB is used for the short-flag, the max_size() is numeric_limits<size_type>::max() / 2.
   // This does not impact the short string representation, since we never need the MSB
   // for representing the size of a short string anyway.
-#if defined(_LIBCPP_BIG_ENDIAN) == defined(_LIBCPP_ABI_ALTERNATE_STRING_LAYOUT)
+#  if defined(_LIBCPP_BIG_ENDIAN) == defined(_LIBCPP_ABI_ALTERNATE_STRING_LAYOUT)
   static const size_type __endian_factor = 2;
-#else
+#  else
   static const size_type __endian_factor = 1;
-#endif
+#  endif
 
 #  ifdef _LIBCPP_ABI_ALTERNATE_STRING_LAYOUT