VS 2017 toolsets now use this flag by default if `UseFullPaths` is
not explicitly set to `false` in the .vcxproj file.  Since there is
no negative form of this flag there is no way for projects to turn
it off through our flag map.  Also, the Makefile and Ninja generators
do not add this flag unless it is explicitly specified by the project.

Teach our generator to set `UseFullPaths` to `false` in VS 2017
unless the project or use has explicitly specified `/FC`.

Fixes: #18261