Fix the _Choose_pocca_v branch in basic_string::operator=(const&)

[achabense]

Fixes #3997; also adds test coverages for non-trivial pointers.
---update---
Fixes another lifetime problem in _Take_contents. Hopefully fixes all the lifetime issue of _Ptr in basic_string. Possibly related to #3949.

Thanks to @frederick-vs-ja for suggesting adding test coverage for non-trivial pointers! Also thanks for implementing VSO_0000000_fancy_pointers, that test helps me a lot

---update (solved with workaround)---
The updated test meets another problem in static_assert(test_sequence<vector>)'s test_sequence_move_ctor... (see #4031 (comment))

[achabense]

_Tidy_init assumes that the pointer is already deactivated. (Perhaps we need more cleanups for the relevant logics in the future.)

[achabense]

emm, for clarity it might be better just to put the correction _Destroy_in_place(_Right_data._Bx._Ptr); at the original place.

Update: (Not planned in this pr)

_Destroy_in_place is typically followed by _Activate_SSO_buffer; however in this function, the _Activate_SSO_buffer call is put off to _Right._Tidy_init(); at the end. This suggests highly flaky maintenance state. I think we need to re-investigate the usages of _Activate_SSO_buffer / _Tidy_init / ... in the codebase.

Especially, I believe there is no need to call _Activate_SSO_buffer at basic_string's constructor, as the _String_val already initialized the buffer in its ctor. However, a lot of basic_string's ctor calls a _Tidy_init, which includes a _Activate_SSO_buffer.

STL/stl/inc/xstring

Line 2284 in 8f67ece

_CONSTEXPR20 _Bxty() noexcept : _Buf() {} // user-provided, for fancy pointers

_Destroy_in_place is closely related to _Activate_SSO_buffer. When switching to SSO mode both function must be called in order. However there are several places that the they are dangerously separated into different function calls. Especially, I've found another place that lacks _Destroy_in_place. (in next comment)
In short, I think we should do _Activate_SSO_buffer and _Destroy_in_place only when switching back to SSO mode, and the two functions are so closely related that they should not be separated into different functions. As this pr shows, the separation can result in maintenance burden and has become a source of bugs.

[achabense]

(I think it's ok not to fix this in this pr)

STL/stl/inc/xstring

Lines 2963 to 2966 in 8f67ece

	if (_My_data._Large_mode_engaged()) {
	_Result._Ptr = _My_data._Bx._Ptr;
	_Result._Actual_allocation_size = _My_data._Myres + 1;
	} else {

It seems this place also lacks _Destroy_in_place. Not caught by the test as it is interaction part with basic_stringbuf.
(Notice that this function also calls _Tidy_init(); at the end.)

STL/stl/inc/xstring

Line 2974 in 8f67ece

_Tidy_init();

[StephanTLavavej]

The product code changes are very clear and the new test coverage provides sufficient confidence that this is correct; I think we can merge this without a second maintainer approval.

[StephanTLavavej]

I'm mirroring this to the MSVC-internal repo - please notify me if any further changes are pushed.

[StephanTLavavej]

Thanks for thoroughly fixing this bug! 🐞 🛠️ ✅