[libc++] Refactor basic_string::__grow_by calls

[philnik777]

This patch introduces a few wrapper functions around __grow_by to make
it clearer what the intention of the different calls is. Later these can
probably be implemented in a simpler manner than __grow_by.

[github-actions]

⚠️ C/C++ code formatter, clang-format found issues in your code. ⚠️

You can test this locally with the following command:

git-clang-format --diff 4562efc674a5b5e052abdfc40047e82a359d0df0 05b23a9df84031c287dbad0952b8a1f39a425b41 --extensions  -- libcxx/include/string

View the diff from clang-format here.

diff --git a/libcxx/include/string b/libcxx/include/string
index 829723ebd5..1dc039821c 100644
--- a/libcxx/include/string
+++ b/libcxx/include/string
@@ -2605,7 +2605,7 @@ void _LIBCPP_CONSTEXPR_SINCE_CXX20 _LIBCPP_HIDE_FROM_ABI basic_string<_CharT, _T
   auto __guard = std::__make_scope_guard(__annotate_new_size(*this));
 
   auto __old_cap = capacity();
-  auto __old_sz =  size();
+  auto __old_sz  = size();
   _LIBCPP_SUPPRESS_DEPRECATED_PUSH
   __grow_by(__old_cap, __new_cap - __old_cap, __old_sz, __n_copy, __n_del, __n_add);
   _LIBCPP_SUPPRESS_DEPRECATED_POP