Avoid compiling CMake itself as C++17 with Clang's MSVC ABI

GUID functions in ATL cause compilation errors with this compiler.
Add the offending case to our check for C++17 support so that it
is not used by default when it does not work.

Source/Checks/cm_cxx17_check.cpp

@@ -2,8 +2,21 @@
 #include <memory>
 #include <unordered_map>
 
+#ifdef _MSC_VER
+#  include <comdef.h>
+#endif
+
 int main()
 {
   std::unique_ptr<int> u(new int(0));
+
+#ifdef _MSC_VER
+  // clang-cl has problems instantiating this constructor in C++17 mode
+  //  error: indirection requires pointer operand ('const _GUID' invalid)
+  //      return *_IID;
+  IUnknownPtr ptr{};
+  IDispatchPtr disp(ptr);
+#endif
+
   return *u;
 }