CSharp: Fix regression in VS project type selection

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

Source/cmGeneratorTarget.cxx

@@ -5227,10 +5227,14 @@ bool cmGeneratorTarget::HasLanguage(std::string const& language,
 {
   std::set<std::string> languages;
   this->GetLanguages(languages, config);
+  // The "exclusive" check applies only to source files and not
+  // the linker language which may be affected by dependencies.
+  if (exclusive && languages.size() > 1) {
+    return false;
+  }
   // add linker language (if it is different from compiler languages)
   languages.insert(this->GetLinkerLanguage(config));
-  return (languages.size() == 1 || !exclusive) &&
-    languages.count(language) > 0;
+  return languages.count(language) > 0;
 }
 
 void cmGeneratorTarget::ComputeLinkImplementationLanguages(

Source/cmGeneratorTarget.h

@@ -366,7 +366,7 @@ public:
 
   // Evaluate if the target uses the given language for compilation
   // and/or linking. If 'exclusive' is true, 'language' is expected
-  // to be the only language used for the target.
+  // to be the only language used in source files for the target.
   bool HasLanguage(std::string const& language, std::string const& config,
                    bool exclusive = true) const;
 

Tests/CSharpLinkToCxx/CMakeLists.txt

@@ -21,3 +21,9 @@ target_link_libraries(CSharpLinkToCxx CLIApp)
 # because it is unmanaged
 add_library(CppNativeApp SHARED cpp_native.hpp cpp_native.cpp)
 target_link_libraries(CSharpLinkToCxx CppNativeApp)
+
+# Link a static C++ library into the CSharp executable.
+# We do not actually use any symbols but this helps cover
+# link language selection.
+add_library(CppStaticLib STATIC cpp_static.cpp)
+target_link_libraries(CSharpLinkToCxx CppStaticLib)

Tests/CSharpLinkToCxx/cpp_static.cpp

@@ -0,0 +1,3 @@
+void cpp_static()
+{
+}