[Issue 21370] New: Missing module in core.stdcpp

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=21370

          Issue ID: 21370
           Summary: Missing module in core.stdcpp
           Product: D
           Version: D2
          Hardware: x86_64
                OS: Linux
            Status: NEW
          Severity: major
          Priority: P1
         Component: druntime
          Assignee: nobody at puremagic.com
          Reporter: jcrapuchettes at gmail.com

In pull request #2723 of druntime, a reference to a module in core.stdcpp
package was added without the actual module.

https://github.com/dlang/druntime/pull/2723/files#diff-6058714bca9002876fe6184b5b2c2f39b68b490de88442b6619f84e304e39e41R132

Using core.stdcpp.memory : unique_ptr on a linux build will cause the compiler
to error on the missing module.

Example code that will cause error:
```
import core.stdcpp.memory : unique_ptr;
void main()
{
    alias up = unique_ptr!char;
}
```

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