[libc++] Fix basic_string::shrink_to_fit for constant evaluation

Currently, when the string shrink into the SSO buffer, the lifetime of
the buffer hasn't begun before copying characters into it, so the
subsequent copy operation raises UB and thus causes constant evaluation
failure.

The existing test coverage seems a bit defective  - `shrink_to_fit` is
called on the copy of string after erasure, not the original string
object.

This PR reorders the `__set_short_size` call, which starts the lifetime
of the SSO buffer,  before the copy operation. Test coverage is achieved
by calling `shrink_to_fit` on the original erased string.

Author: frederick-vs-ja

libcxx/include/string

@@ -3434,8 +3434,8 @@ inline _LIBCPP_CONSTEXPR_SINCE_CXX20 void basic_string<_CharT, _Traits, _Allocat
 
   if (__fits_in_sso(__target_capacity)) {
     __annotation_guard __g(*this);
-    traits_type::copy(std::__to_address(__get_short_pointer()), std::__to_address(__ptr), __size + 1);
     __set_short_size(__size);
+    traits_type::copy(std::__to_address(__get_short_pointer()), std::__to_address(__ptr), __size + 1);
     __alloc_traits::deallocate(__alloc_, __ptr, __cap);
     return;
   }

libcxx/test/std/strings/basic.string/string.capacity/shrink_to_fit.pass.cpp

@@ -19,7 +19,7 @@
 #include "test_macros.h"
 
 template <class S>
-TEST_CONSTEXPR_CXX20 void test(S s) {
+TEST_CONSTEXPR_CXX20 void test(S& s) {
   typename S::size_type old_cap = s.capacity();
   S s0                          = s;
   s.shrink_to_fit();