5.12.0 and later permit -C on the #! line with similar rules to -T

5.12.0 and later permit -C on the #! line with similar rules to -T

Whilst this isn't as totally flexible as it might be, it's good enough to
remove the todo entry, which was added when -C was forbidden on the #! line.
However, making -C parsing on the #! more flexible probably requires
removing the "feature" that a C<#line 1> directive triggers re-processing
of #! directives.