Direct2D QPA: Fix build with clang-cl.

Remove overzealous constexpr triggering errors like:

in file included from .\qwindowsdirect2dpaintengine.cpp:44:
./qwindowsdirect2dhelpers.h(58,37) :  error: constexpr function never produces a constant expression [-Winvalid-constexpr]
Q_DECL_CONSTEXPR inline D2D1_RECT_F to_d2d_rect_f(const QRectF &qrect)
                                    ^
./qwindowsdirect2dhelpers.h(60,12) :  note: non-constexpr function 'RectF' cannot be used in a constant expression
    return D2D1::RectF(qrect.x(), qrect.y(), qrect.x() + qrect.width(), qrect.y() + qrect.height());
           ^
C:\Program Files (x86)\Windows Kits\10\include\10.0.10586.0\um\d2d1helper.h(160,5) :  note: declared here
    RectF(
    ^

Task-number: QTBUG-50860
Change-Id: I0bb4d17b7a7db69c5cc7e286ef0b8ff104f72b2e
Reviewed-by: Joerg Bornemann <joerg.bornemann@theqtcompany.com>