CSharp: Fix regression in VS project type selection for custom target

A target created by `add_custom_target` should always be a `.vcxproj`
file even if it has `.cs` sources involved in custom commands and such.
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) does not check the target type
and so is not a suitable check for deciding the project file extension.

The `HasLanguage` method was an attempt at an abstraction that turns
out not to work very well.  Replace it with a dedicated `IsCSharpOnly`
method that considers the target type, sources, and non-transitive
`LINKER_LANGUAGE`.

Fixes: #18515

1 files changed, 1 insertions, 1 deletions

diff --git a/Source/cmGlobalVisualStudio71Generator.cxx b/Source/cmGlobalVisualStudio71Generator.cxx
index 0b086b0..ba12fac 100644
--- a/Source/cmGlobalVisualStudio71Generator.cxx
+++ b/Source/cmGlobalVisualStudio71Generator.cxx
@@ -98,7 +98,7 @@ void cmGlobalVisualStudio71Generator::WriteProject(std::ostream& fout,
     ext = ".vfproj";
     project = "Project(\"{6989167D-11E4-40FE-8C1A-2192A86A7E90}\") = \"";
   }
-  if (t->HasLanguage("CSharp", "")) {
+  if (t->IsCSharpOnly()) {
     ext = ".csproj";
     project = "Project(\"{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}\") = \"";
   }