VS: Split flag table between v140 and v141 toolsets

In commit a8936656 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 (closed)