[libcxx] Remove Redundant Reset in ~basic_string

8dae17b refactors basic_string for more
code reuse. This makes sense in most cases, but has performance overhead
in the case of ~basic_string. The refactoring of ~basic_string to call
__reset_internal_buffer() added a redundant (inside the destructor)
reset of the object, which the optimizer is unable to optimize away in
many cases. This patch prevents a ~1% regression we observed on an
internal workload when applying the original refactoring. This does
slightly pessimize the code readability, but I think this change is
worth it given the performance impact.

I'm hoping to add a benchmark(s) to the upstream libc++ benchmark suite
around string construction/destruction to ensure that this case does not
regress as it seems common in real world applications. I will put up a
separate PR for that when I figure out a reasonable way to write it.

Author: boomanaiden154

libcxx/include/string

@@ -1206,7 +1206,13 @@ public:
   }
 #  endif // _LIBCPP_CXX03_LANG
 
-  inline _LIBCPP_CONSTEXPR_SINCE_CXX20 ~basic_string() { __reset_internal_buffer(); }
+  inline _LIBCPP_CONSTEXPR_SINCE_CXX20 ~basic_string() {
+    // We do not use __reset_internal_buffer() here to avoid the overhead of
+    // resetting the object.
+    __annotate_delete();
+    if (__is_long())
+      __alloc_traits::deallocate(__alloc_, __get_long_pointer(), __get_long_cap());
+  }
 
   _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 operator __self_view() const _NOEXCEPT {
     return __self_view(typename __self_view::__assume_valid(), data(), size());