Implement QLibrary::PreventUnloadHint for Windows

It's interesting that the HMODULE/HINSTANCE pointer points to something
in the actual module that got loaded, so we can use its value as
"address of something in the module" for GetModuleHandleEx. The "PIN"
flag tells the Windows DLL loader to never unload.

Change-Id: Ifea6e497f11a461db432ffff1449a2169eb6293e
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@qt.io>
Reviewed-by: Kai Koehne <kai.koehne@qt.io>
Reviewed-by: Joerg Bornemann <joerg.bornemann@theqtcompany.com>

src/corelib/plugin/qlibrary.cpp

@@ -1096,8 +1096,6 @@ QString QLibrary::errorString() const
     to the library \c shr_64.o in the archive file named \c libGL.a. This
     is only supported on the AIX platform.
 
-    Setting PreventUnloadHint will only apply on Unix platforms.
-
     The interpretation of the load hints is platform dependent, and if
     you use it you are probably making some assumptions on which platform
     you are compiling for, so use them only if you understand the consequences

src/corelib/plugin/qlibrary_win.cpp

@@ -148,6 +148,16 @@ bool QLibraryPrivate::load_sys()
         else
             qualifiedFileName = dir.filePath(moduleFileName);
 #endif // !Q_OS_WINRT
+
+        if (loadHints() & QLibrary::PreventUnloadHint) {
+            // prevent the unloading of this component
+            HMODULE hmod;
+            bool ok = GetModuleHandleEx(GET_MODULE_HANDLE_EX_FLAG_PIN |
+                                        GET_MODULE_HANDLE_EX_FLAG_FROM_ADDRESS,
+                                        reinterpret_cast<const wchar_t *>(pHnd),
+                                        &hmod);
+            Q_ASSERT(!ok || hmod == pHnd);
+        }
     }
     return (pHnd != 0);
 }