`<sstream>`, `<syncstream>`: Buffer types assume that allocated pointers are not modified by users

Describe the bug

Currently, MSVC STL's basic_stringbuf relies on that the eback() pointer points to the beginning of the allocated buffer.
https://github.com/microsoft/STL/blob/e077eb66871626d93f50d329e8fa19cefa80aeb3/stl/inc/sstream#L495-L497

Likewise, basic_syncbuf requires pbase() to behave the same way.
https://github.com/microsoft/STL/blob/e077eb66871626d93f50d329e8fa19cefa80aeb3/stl/inc/syncstream#L247

However, the pointers returned by these functions can be modified by users via setg/setp functions, which means these conditions can't be considered as invariants maintained by the implementation.

Command-line test case

https://github.com/microsoft/STL/blob/e077eb66871626d93f50d329e8fa19cefa80aeb3/tests/libcxx/expected_results.txt#L1103-L1107

Expected behavior

These tests pass.

STL version

https://github.com/microsoft/STL/commit/e077eb66871626d93f50d329e8fa19cefa80aeb3

(Probably in all versions where basic_stringbuf or basic_syncbuf is available.)

Additional context

Not sure whether this can be fixed without breaking ABI. #4239 may be related, but doesn't fix any part of this issue.

Inspect the referenced implementations in stl/inc/sstream and stl/inc/syncstream, then review the command-line test case and its entry in tests/libcxx/expected_results.txt. Determine how user-modified eback() and pbase() pointers affect the existing assumptions, while checking the ABI concern noted in the issue. Done means the linked tests pass without relying on those pointers remaining unchanged.