cmake: Comment why we ignore the include-what-you-use return code

The include-what-you-use tool always returns non-zero to indicate that
it did not actually produce an object file as Clang would from the same
command line.  Add a comment explaining that this is why we ignore its
return code.  Also update our `pseudo_iwyu` test suite tool to always
exit with an error too.

Source/cmcmd.cxx

@@ -327,6 +327,7 @@ int cmcmd::ExecuteCMakeCommand(std::vector<std::string>& args)
         iwyu_cmd.insert(iwyu_cmd.end(), orig_cmd.begin() + 1, orig_cmd.end());
 
         // Run the iwyu command line.  Capture its stderr and hide its stdout.
+        // Ignore its return code because the tool always returns non-zero.
         std::string stdErr;
         if (!cmSystemTools::RunSingleCommand(iwyu_cmd, CM_NULLPTR, &stdErr,
                                              &ret, CM_NULLPTR,

Tests/RunCMake/pseudo_iwyu.c

@@ -3,5 +3,6 @@
 int main(void)
 {
   fprintf(stderr, "should add these lines:\n#include <...>\n");
-  return 0;
+  /* include-what-you-use always returns failure */
+  return 1;
 }