This causes various runtime failures, as reported in #168628.

This reverts both #165223 and #167779

The optimized version of xsgetn for basic_filebuf added in #165223 has
an issue where if the reads come from both the buffer and the
filesystem it returns the wrong number of characters. This patch should
address the issue.

```
Benchmark         old       new    Difference    % Difference
-----------  --------  --------  ------------  --------------
bm_read       2468.45    736.27      -1732.18         -70.17%
```

Apple dropped support for some older platforms, so we can also remove
the annotations for them.
See https://developer.apple.com/support/xcode/ for the supported
versions.

```
----------------------------
Benchmark       old      new
----------------------------
bm_write    1382 ns   521 ns
```

This patch makes the `__failed` lambda a member function on `fstream`.
This fixes two LLDB expression evaluation test failures that got
introduced with https://github.com/llvm/llvm-project/pull/147389:
```
16:22:51  ********************
16:22:51  Unresolved Tests (2):
16:22:51    lldb-api :: commands/expression/import-std-module/list-dbg-info-content/TestDbgInfoContentListFromStdModule.py
16:22:51    lldb-api :: commands/expression/import-std-module/list/TestListFromStdModule.py
```

The expression evaluator is asserting in the Clang parser:
```
Assertion failed: (capture_size() == Class->capture_size() && "Wrong number of captures"), function LambdaExpr, file ExprCXX.cpp, line 1277.
PLEASE submit a bug report to https://github.com/llvm/llvm-project/issues/ and include the crash backtrace.
```

Ideally we'd figure out why LLDB is falling over on this lambda. But to
unblock CI for now, make this a member function.

In the long run we should figure out the LLDB bug here so libc++ doesn't
need to care about whether it uses lambdas like this or not.

In rare circumstances, the invariants could fail to be restored.

Refactor the function to streamline the logic so it matches the
specification in [filebuf.virtuals] more closely. In particular, avoid
modifying the put area pointers when we loop around after a partial
codecvt conversion.

Note that we're technically not up-to-spec in this implementation, since
the Standard says that we shouldn't try more than once after a partial
codecvt conversion. However, this refactoring attempts not to change any
functionality.

This reverts commit c861fe8a71e64f3d2108c58147e7375cd9314521.

Unfortunately, this use of hidden visibility attributes causes
user-defined specializations of standard-library types to also be marked
hidden by default, which is incorrect. See discussion thread on #131156.

...and also reverts the follow-up commits:

Revert "[libc++] Add explicit ABI annotations to functions from the block runtime declared in <__functional/function.h> (#140592)"
This reverts commit 3e4c9dc299c35155934688184319d391b298fff7.

Revert "[libc++] Make ABI annotations explicit for windows-specific code (#140507)"
This reverts commit f73287e623a6c2e4a3485832bc3e10860cd26eb5.

Revert "[libc++][NFC] Replace a few "namespace std" with the correct macro (#140510)"
This reverts commit 1d411f27c769a32cb22ce50b9dc4421e34fd40dd.