Update the logic that converts a `try_compile` result from a cache
entry to a normal variable to tolerate an existing normal variable
under CMP0126 NEW behavior. Otherwise the `try_compile` result
is ignored because CMake uses the false value of the normal variable,
and CMake incorrectly reports that the compiler does not work.
This went unnoticed for some languages (e.g. C and CXX) because the
check for a working compiler is skipped if ABI detection works.
It does affect other languages (e.g. CSharp).
Fixes: #22423
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
