In commit v3.7.0-rc1~156^2~1 (VS: Update v140 flag tables from VS 15
MSBuild files, 2016-09-02) we extended the v140 flag table with values
from the v141 toolset that comes with VS 15. However, the v140 toolset
that comes with VS 14 does not have all of these entries and so the
flags just need to be passed without special mapping. In order to
support both toolsets, split our CL flag table into separate copies for
each version and switch off the toolset name.
Closes: #16352
Brad King