CompilerId: Fix default extensions check for Clang targeting MSVC ABI
In commit fc3a1cbd (CompilerID: Compiler extensions default detection, 2021-05-29, v3.22.0-rc1~52^2~2), 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.