Fix qtbase build for clang-cl with MSVC 2017 15.8

Move the definition of _ENABLE_EXTENDED_ALIGNED_STORAGE
to msvc-desktop.conf so that it becomes effective
for all compilers using MSVC (icc, clang-cl).

Task-number: QTBUG-50804
Change-Id: I5ff612cc0f5a712b855925f9bcf645e578e80504
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>