[Bug #20468] Fix safe navigation in `for` variable

[Bug #20280]

Backport of #10014.

[Bug #20180]

Backports #9624.

merge revision(s) 596db9c1f486d6609a4e97d82c8c71b54609fb6f: [Backport #20090]

	[Feature #19370] Blocks without anonymous parameters should not
	 affect

	---
	 parse.y                  | 4 ++--
	 test/ruby/test_syntax.rb | 7 +++++++
	 2 files changed, 9 insertions(+), 2 deletions(-)

At the method definition, the local scope that saves the context of
the numbered parameters needs to be pushed before saving.

Since Ruby 2.4, `return` is supported at toplevel.  This makes `eval "return"`
also supported at toplevel.

This mostly uses the same tests as direct `return` at toplevel, with a couple
differences:

`END {return if false}` is a SyntaxError, but `END {eval "return" if false}`
is not an error since the eval is never executed. `END {return}` is a
SyntaxError, but `END {eval "return"}` is a LocalJumpError.

The following is a SyntaxError:

```ruby
class X
nil&defined?0--begin e=no_method_error(); return; 0;end
end
```

However, the following is not, because the eval is never executed:

```ruby
class X
nil&defined?0--begin e=no_method_error(); eval "return"; 0;end
end
```

Fixes [Bug #19779]

https://bugs.ruby-lang.org/issues/18980