Cleanups 8: Deprecate <strstream> classes instead of the entire header

[StephanTLavavej]

Deprecating the entire <strstream> header is problematic for the proposed Standard Library Modules, as <strstream> is still Standard C++23 and must be part of import std;.

This change makes it so that merely including <strstream> no longer emits a deprecation warning (via the two-typedef trick). Instead, the deprecation is applied to the 4 classes within. The warning message is still appropriate:

STL/stl/inc/yvals_core.h

Lines 787 to 790 in 314f65f

	#define _CXX17_DEPRECATE_STRSTREAM \
	[[deprecated("warning STL4005: <strstream> is deprecated in C++17. " \
	"You can define _SILENCE_CXX17_STRSTREAM_DEPRECATION_WARNING " \
	"or _SILENCE_ALL_CXX17_DEPRECATION_WARNINGS to acknowledge that you have received this warning.")]]

We must wrap the header in _STL_DISABLE_DEPRECATED_WARNING / _STL_RESTORE_DEPRECATED_WARNING as the classes mention each other.

I am not deprecating the non-Standard swap() overloads - there is little point in doing so when the Standard classes are already marked.

I am also detaching comments on the classes.

Then, this allows _SILENCE_CXX17_STRSTREAM_DEPRECATION_WARNING to be removed in various places that were merely including <strstream> but not attempting to use its machinery. (Silencing is still necessary in tests/std/tests/P1502R1_standard_library_header_units and tests/tr1/tests/strstream which exercise the machinery.)

Note that the "deprecate the entire header" technique is still being applied to the <ccomplex>, <cstdalign>, <cstdbool>, and <ctgmath> headers, as they are super useless, removed in C++20, and I won't be adding them to Standard Library Modules. (<experimental/filesystem> and <hash_meow> have the even stronger #error-unless-escape-hatch technique; they are non-Standard so they are also unchanged here.)

[StephanTLavavej]

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