If given a toolset that does not have an explicit mapping in
cmVisualStudio10ToolsetOptions, check for a json flag file using the
toolset name before trying the default toolset for the generator.
Add these (currently unused) tables in preparation for `.csproj`
generation support. Populate the tables for every version with a set of
initial values that work well for me with VS 12 and VS 14. Later we may
need to generate them more thoroughly from MSBuild `.xml` files.
MSBuild interprets the `.vcxproj` content based on the `PlatformToolset`
setting, so our reverse mapping needs to be based on that setting too.
For VS 2010 and above, choose the flag map to match the toolset name
rather than the generator VS version.
Issue: #16153
