Use CurrentImplementation::current() instead of VC_IMPL
Use CurrentImplementation::current() instead of VC_IMPL

* Favor a C++ constant expression over a macro here. This also makes the
AVX2 tests link, so the macro seems to get messed up somewhere.

Refs: gh-11
Signed-off-by: Matthias Kretz <372ea4eca737a13cd010ee806d4ce7b98ee71550@kde.org>

VcDevel/Vc,VcDevel/Vc,chr-engwer/Vc,chr-engwer/Vc,chr-engwer/Vc,VcDevel/Vc,VcDevel/Vc