VS: Fix /MAP:mapfile flag mapping (#14282)
We have two mappings for the "/MAP" flag. The first does not care whether there is a value and activates the GenerateMapFile boolean setting. The second takes a value and puts it in the MapFileName setting. The latter must treat the ":" as part of the flag. This is similar to commit 8ae66bf4 (Fix optionally-valued booleans in VS 10 flag table, 2009-10-23).