[Issue 20022] New: POSIX: extern(C++, `namespace`) does not apply namespace to enum

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=20022

          Issue ID: 20022
           Summary: POSIX: extern(C++, `namespace`) does not apply
                    namespace to enum
           Product: D
           Version: D2
          Hardware: All
                OS: Linux
            Status: NEW
          Severity: regression
          Priority: P1
         Component: dmd
          Assignee: nobody at puremagic.com
          Reporter: pro.mathias.lang at gmail.com

```
extern(C++, namespace) enum FooBar { A = 1, }
extern(C++, `namespace`) enum FooBar2 { A = 1, }

pragma(msg, FooBar.mangleof);
pragma(msg, FooBar2.mangleof);
```

DMD 2.086.1:
```
E3foo9namespace6FooBar
E3foo9namespace7FooBar2
```

DMD 2.087.0:
```
E3foo9namespace6FooBar
E3foo7FooBar2
```

Most likely introduced by my refactoring:
https://github.com/dlang/dmd/pull/10021

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