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.

1 files changed, 2 insertions, 1 deletions

diff --git a/Tests/RunCMake/pseudo_iwyu.c b/Tests/RunCMake/pseudo_iwyu.c
index 1e25de7..c761741 100644
--- a/Tests/RunCMake/pseudo_iwyu.c
+++ b/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;
 }