QCoreApplication/Win: add a better way for ActiveQt to override qAppFileName

Right now, the DLLMain in qaxserverdll.cpp calls GetModuleFileName() on
the hInstance for the DLL and passes the path to QCoreApplication to
override the path. This adds a way for it to simply pass the hInstance
handle, which will automatically cause qAppFileName() &
QCoreApplication::applicationFilePath() return the DLL's path, which in
turn influences the default QCoreApplication::libraryPaths().

Pick-to: 6.9
Change-Id: I3cc172288c54c3938566fffdfb62985000ce2e9c
Reviewed-by: Oliver Wolff <oliver.wolff@qt.io>

src/corelib/kernel/qcoreapplication_p.h

@@ -130,6 +130,7 @@ public:
     std::unique_ptr<char *[]> origArgv;
 
     bool consoleAllocated = false;
+    static void *mainInstanceHandle;    // HINSTANCE without <windows.h>
 #endif
     void appendApplicationPathToLibraryPaths(void);
 

src/corelib/kernel/qcoreapplication_win.cpp

@@ -20,6 +20,9 @@ QT_BEGIN_NAMESPACE
 
 using namespace Qt::StringLiterals;
 
+// By default, we get the path to the host .exe. ActiveQt can override this
+// with the component's DLL.
+Q_CONSTINIT void *QCoreApplicationPrivate::mainInstanceHandle = nullptr;
 QString qAppFileName()                // get application file name
 {
     /*
@@ -45,7 +48,8 @@ QString qAppFileName()                // get application file name
     do {
         size += MAX_PATH;
         space.resize(int(size));
-        v = GetModuleFileName(NULL, space.data(), DWORD(space.size()));
+        auto hInstance = reinterpret_cast<HINSTANCE>(QCoreApplicationPrivate::mainInstanceHandle);
+        v = GetModuleFileName(hInstance, space.data(), DWORD(space.size()));
     } while (Q_UNLIKELY(v >= size));
 
     return QString::fromWCharArray(space.data(), v);