Demo: `compile(sample, "<sample>", "exec", PyCF_ONLY_AST|0x1000)`
This will crash if type comments are present.

It doesn't *work* yet -- type comments cause crashes in ast.c and type
ignore comments aren't passed on to Module yet.

This works, it just needs tests

This fixes

if 1:
foo()

but does nothing about

def f():
foo()

The latter still reports 'SyntaxError: invalid syntax'.

So this is not a real solution, just a stop-gap measure until I find a better solution.

Also:
- remove redundant 'c' argument from new_type_comment()
- double-check that TYPE_COMMENT in a suite is followed by NL

pythonGH-11645 added the fourth option for `mode` argument; updating the comment accordingly.