Don't load Clang-CXX from AppleClang-CXX.

The Clang-CXX module has version-specific checks, and the version
system for AppleClang is not the same as Clang.
Only populate Clang features if CMP0025 is NEW.

That policy determines whether the version corresponds to the
upstream versioning of Clang or the different versioning system
of AppleClang.
Merge topic 'target_compile_features' into next

51291c6 Only populate Clang features if CMP0025 is NEW.
1026b96 Don't load Clang-CXX from AppleClang-CXX.