VS: Fix mapping of `-Qspectre-` flag
The mapping for this flag was added by !2856 (merged). However, it did not do anything because the special logic added by
!2462 (merged) to move the
SpectreMitigation element from
ClCompile to the top level only handled the presence of the setting
and not its value. Extend the special logic to carry the value too.
Fixes: #19535 (closed)