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.
53 jobs for !6949 with default-extensions in 16 minutes and 15 seconds (queued for 3 seconds)
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