Fix incorrect offense for Style/EmptyLineAfterGuardClause

[koic]

Follow up of #5720 (comment).

This PR fixes incorrect offense location for Style/EmptyLineAfterGuardClause when guard clause is after heredoc argument.

Reproduction code

def foo
  raise ArgumentError, <<-MSG unless path
    Must be called with mount point
  MSG
  bar
end

Expected behavior

% rubocop /tmp/example.rb --only Style/EmptyLineAfterGuardClause
Inspecting 1 file
C

Offenses:

/tmp/example.rb:4:1: C: Style/EmptyLineAfterGuardClause: Add empty line
after guard clause.
  MSG
^^^^^

1 file inspected, 1 offense detected

Actual behavior and Steps to reproduce the problem

% rubocop /tmp/example.rb --only Style/EmptyLineAfterGuardClause
Inspecting 1 file
C

Offenses:

/tmp/example.rb:2:3: C: Style/EmptyLineAfterGuardClause: Add empty line
after guard clause.
  raise ArgumentError, <<-MSG unless path
  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

1 file inspected, 1 offense detected

RuboCop version

% rubocop -V
0.54.0 (using Parser 2.5.0.5, running on ruby 2.5.1 x86_64-darwin17)

Also, this PR will fix bugs included in changes at #5720.

---

Before submitting the PR make sure the following are checked:

• Wrote good commit messages.
• Commit message starts with [Fix #issue-number] (if the related issue exists).
• Feature branch is up-to-date with master (if not - rebase it).
• Squashed related commits together.
• Added tests.
• Added an entry to the Changelog if the new code introduces user-observable changes. See changelog entry format.
• The PR relates to only one subject with a clear title
  and description in grammatically correct, complete sentences.
• Run rake default or rake parallel. It executes all tests and RuboCop for itself, and generates the documentation.