CompilerId: Fix default extensions check for Clang targeting MSVC ABI

In !6177 (merged), extensions were assumed to be OFF for Clang targeting the MSVC ABI. However, the clang and clang++ tools both seem to have extensions enabled by default even when targeting the MSVC ABI.

This can be observed with the RunCMake.CompileFeatures test. It fails with the GNU-like clang/clang++ front-end, but removing the above special case makes it pass. The test passes either way with the MSVC-like clang-cl front-end.

Backport: release