[ATL] Implement CSimpleStringT::Preallocate (reactos#6574)

Used in `ATL::CStringW` and `ATL::CStringA`, for example:

```
ATL::CStringW cswItemText = "";
cswItemText.Preallocate(64);

SendDlgItemMessageW(pdis->CtlID, LB_GETTEXT, pdis->itemID, reinterpret_cast<LPARAM>(cswItemText.GetBuffer()));
cswItemText.ReleaseBuffer();
```

This public function is used to allocate memory for the string via private PrepareWrite, but it's missing somehow. Now it shouldn't be.

sdk/lib/atl/atlsimpstr.h

@@ -373,6 +373,11 @@ class CSimpleStringT
         return m_pszData;
     }
 
+    void Preallocate(_In_ int nLength)
+    {
+        PrepareWrite(nLength);
+    }
+
     void ReleaseBufferSetLength(_In_ int nNewLength)
     {
         ATLASSERT(nNewLength >= 0);