[libc++] Avoid spuriously unsupporting the new FTM tests

The new FTM tests contain text that they validate against to check
the output of the FTM generation script. However, that text lexically
contains the characters `// UNSUPPORTED: <...>`, which leads Lit to
make the whole test unsupported under these conditions. To prevent
that from happening, an `# END.` block can be used to prevent Lit
from looking further into the file for directives.

Author: ldionne

libcxx/test/libcxx/feature_test_macro/generate_header_test.sh.py

@@ -7,6 +7,7 @@
 # ===----------------------------------------------------------------------===##
 
 # RUN: %{python} %s %{libcxx-dir}/utils %{libcxx-dir}/test/libcxx/feature_test_macro/test_data.json %t/tests
+# END.
 
 import os
 import sys