[ruby-core:101837] [Ruby master Bug#17495] Make args info for :RubyVM:AST to available on endless method without parens

Issue #17495 has been reported by pocke (Masataka Kuwabara).

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Bug #17495: Make args info for :RubyVM:AST to available on endless method without parens
https://bugs.ruby-lang.org/issues/17495

* Author: pocke (Masataka Kuwabara)
* Status: Open
* Priority: Normal
* ruby -v: $ ruby -v ruby 3.1.0dev (2020-12-31T03:30:13Z master 62450e0acf) [x86_64-linux]
* Backport: 2.5: UNKNOWN, 2.6: UNKNOWN, 2.7: UNKNOWN
----------------------------------------

Problem
===

Arguments information is missing for endless method without parens.
For example:

```ruby
# ok
pp RubyVM::AbstractSyntaxTree.parse(<<~RUBY).children[2]
  def x() = 42
RUBY
# => (DEFN@1:0-1:12
#     mid: :x
#     body:
#       (SCOPE@1:0-1:12
#        tbl: []
#        args:
#          (ARGS@1:5-1:6
#           pre_num: 0
#           pre_init: nil
#           opt: nil
#           first_post: nil
#           post_num: 0
#           post_init: nil
#           rest: nil
#           kw: nil
#           kwrest: nil
#           block: nil)
#        body: (LIT@1:10-1:12 42)))
```


```ruby
# ok
pp RubyVM::AbstractSyntaxTree.parse(<<~RUBY).children[2]
  def x() 42 end
RUBY
# => (DEFN@1:0-1:14
#     mid: :x
#     body:
#       (SCOPE@1:0-1:14
#        tbl: []
#        args:
#          (ARGS@1:5-1:6
#           pre_num: 0
#           pre_init: nil
#           opt: nil
#           first_post: nil
#           post_num: 0
#           post_init: nil
#           rest: nil
#           kw: nil
#           kwrest: nil
#           block: nil)
#        body: (LIT@1:8-1:10 42)))
```

```ruby
# It has a problem, the `args` is nil
pp RubyVM::AbstractSyntaxTree.parse(<<~RUBY).children[2]
  def x = 42
RUBY
# => (DEFN@1:0-1:10
#     mid: :x
#     body: (SCOPE@1:0-1:10 tbl: [] args: nil body: (LIT@1:8-1:10 42)))
```

It causes an error if a program expects `args` node exists.
For example: https://github.com/ruby/rbs/issues/551

Solution Proposal
====

A pull request is open on GitHub: https://github.com/ruby/ruby/pull/4016





I referred to the normal method definition's implementation.  https://github.com/ruby/ruby/blob/48cf548beb8d641d7dd2cb05f8470526eff7661e/parse.y#L5138-L5142


 I am a very beginner at parse.y, so I'm not sure this solution is good. If something wrong, please tell me advice 



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