[libc++] Remove basic_string::__clear_and_shrink (#126050)

`__clear_and_shrink` is only used in a single place and does more work
than actually required.

Author: philnik777

libcxx/include/string

@@ -1879,8 +1879,6 @@ public:
 
   _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 bool __invariants() const;
 
-  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 void __clear_and_shrink() _NOEXCEPT;
-
 private:
   _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 _LIBCPP_STRING_INTERNAL_MEMORY_ACCESS bool
   __is_long() const _NOEXCEPT {
@@ -2189,7 +2187,11 @@ private:
       __alloc_ = __str.__alloc_;
     else {
       if (!__str.__is_long()) {
-        __clear_and_shrink();
+        if (__is_long()) {
+          __annotate_delete();
+          __alloc_traits::deallocate(__alloc_, __get_long_pointer(), capacity() + 1);
+          __rep_ = __rep();
+        }
         __alloc_ = __str.__alloc_;
       } else {
         __annotate_delete();
@@ -3820,18 +3822,6 @@ inline _LIBCPP_CONSTEXPR_SINCE_CXX20 bool basic_string<_CharT, _Traits, _Allocat
   return true;
 }
 
-// __clear_and_shrink
-
-template <class _CharT, class _Traits, class _Allocator>
-inline _LIBCPP_CONSTEXPR_SINCE_CXX20 void basic_string<_CharT, _Traits, _Allocator>::__clear_and_shrink() _NOEXCEPT {
-  clear();
-  if (__is_long()) {
-    __annotate_delete();
-    __alloc_traits::deallocate(__alloc_, __get_long_pointer(), capacity() + 1);
-    __rep_ = __rep();
-  }
-}
-
 // operator==
 
 template <class _CharT, class _Traits, class _Allocator>