[libc++] Fix basic_string::shrink_to_fit for constant evaluation (#142712)

Currently, when the string shrink into the SSO buffer, the `__rep_.__s`
member isn't activated before the `traits_type::copy` call
yet, so internal `__builtin_memmove` call writing to the buffer causes
constant evaluation failure. The existing test coverage seems a bit
defective and doesn't cover this case - `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();