VS: Introduce IsInSolution to check whether a target is in sln file

Previously, different versions of VS Generators checked whether a target
was in .sln file or not by checking whether the target was to be written
to build system or not.

As we move `ZERO_CHECK.vcxproj` to `.proj`, we want to exclude those
files from being written to `.sln` files too. This commit introduces
`IsInSolution()` at `cmGlobalVisualStudioGenerator` level which can be
customized at specific versioned Generator when needed.