A that target contains only `.cs` sources should be generated as a
`.csproj` project even if it links to non-CSharp static libraries.
The latter case was broken by refactoring in commit v3.12.0-rc1~160^2~7
(remove TargetIsCSharpOnly() and use methods from cmGeneratorTarget,
2018-03-19). The reason is that the `HasLanguage` method added by
commit v3.12.0-rc1~239^2~6 (cmGeneratorTarget: add HasLanguage() as
wrapper for GetLanguages(), 2018-03-19) enforces its "exclusive" check
on the combined set of source file languages and the link language.
To restore the original `TargetIsCSharpOnly` semantics, update
`HasLanguage` to enforce exclusiveness only on the list of sources.
Fixes: #18239
Since commit v3.9.0-rc4~4^2 (Vs: allow CSharp targets to be linked to
CXX targets, 2017-06-20) CSharp targets get `ProjectReference` entries
to their dependencies. This causes VS to also reference the
dependency's output assembly by default, which is incorrect for
non-managed targets.
Fix this by setting `ReferenceOutputAssembly` to `false` for targets
that can't provide output assemblies. Unmanaged C++ targets (shared
libs & executables) can still be referenced and a warning will be shown
in the IDE but the build will not break anymore.
Fixes: #17172
