VS: Fix Fortran target type selection when linking C++ targets

Since commit 2c9f3578 (VS: Decide project type by linker lang as
fallback, 2017-03-30, v3.9.0-rc1~340^2) we consider the linker language
when detecting whether to generate a `.vfproj` or `.vcxproj` file.
However, this could cause C-only projects to become `.vfproj` files if
they link to Fortran projects.  Instead we should consider only the
`LINKER_LANGUAGE` property on the target itself.  This approach is
already used for CSharp.  It allows project code to specify the project
file type for a target with no sources but does not allow linked targets
to affect it.

Fixes: #18687
10 jobs for vs-fortran-target-check
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