<syncstream>: fix std::osyncstream memory leak

[fsb4000]

Fixes #2760

[fsb4000]

https://docs.microsoft.com/en-us/visualstudio/debugger/finding-memory-leaks-using-the-crt-library?view=vs-2022#enable-memory-leak-detection

To enable all the debug heap functions, include the following statements in your C++ program, in the following order:

#define _CRTDBG_MAP_ALLOC
#include <stdlib.h>
#include <crtdbg.h>

It worked OK with clang-format order but maybe the order is important.

[StephanTLavavej]

I pushed significant test changes, verifying them in three ways:

1. If I comment out the product code fix, the test fails with:

   Assertion failed: _CrtMemDifference(&diff, &start, &end) == 0, file D:\GitHub\STL\tests\std\tests\GH_002760_syncstream_memory_leak\test.cpp, line 25
   abort() has been called--
   

   This demonstrates that the leak checks are still working.

2. Without the REQUIRES comment, 17 configurations pass. This demonstrates that the internal test harness will pass. [[maybe_unused]] was necessary to avoid release mode complaining about the variables.

3. With the REQUIRES comment, 8 configurations are unsupported while 9 pass. The pattern of UNSUPPORTED and PASS configurations exactly corresponds to the release and debug pattern in the matrix.

[StephanTLavavej]

I pushed a product code change for tidying consistently - the tests pass, and I'm reasonably confident it's correct, but lower confidence than usual since iostreams is my magical weak point. Please double check! 😸

[strega-nil-ms]

Thanks!

[StephanTLavavej]

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

[StephanTLavavej]

Thanks for finding the source of this memory leak and fixing it! 💧 🔧 😻