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