[libcxx][fstream][NFC] Make __failed helper lambda a member function (#149390)

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.

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