Fix an incorrect auto-correct for Style/IfUnlessModifier

This PR fixes the following incorrect auto-correct for `Style/IfUnlessModifier`
when using a method with heredoc argument.

```console
% cat .rubocop.yml
Layout/LineLength:
  Max: 80

% cat example.rb
fooooooooooooooooooooooooooooooooooooooooooooooooooooooooooo(<<~EOS) if condition
  string
EOS

% bundle exec rubocop --only Style/IfUnlessModifier -a
(snip)

Inspecting 1 file
C

Offenses:

example.rb:1:70: C: [Corrected] Style/IfUnlessModifier: Modifier form of
if makes the line too long.
fooooooooooooooooooooooooooooooooooooooooooooooooooooooooooo(<<~EOS) if condition
                                                                     ^^

1 file inspected, 1 offense detected, 1 offense corrected
```

## Before

Auto-corrected to unexpected invalid syntax code.

```console
% cat example.rb
if condition
  fooooooooooooooooooooooooooooooooooooooooooooooooooooooooooo(<<~EOS)
end
  string
EOS

% ruby -c example.rb
example.rb:2: syntax error, unexpected end-of-input, expecting `end'
```

## After

Auto-corrected to expected valid syntax code.

```console
% cat example.rb
if condition
  fooooooooooooooooooooooooooooooooooooooooooooooooooooooooooo(<<~EOS)
    string
  EOS
end
```

changelog/fix_incorrect_autocorrect_for_if_unless_modifier.md

@@ -0,0 +1 @@
+* [#9690](https://github.com/rubocop/rubocop/pull/9690): Fix an incorrect auto-correct for `Style/IfUnlessModifier` when using a method with heredoc argument. ([@koic][])

lib/rubocop/cop/style/if_unless_modifier.rb

@@ -38,6 +38,7 @@ class IfUnlessModifier < Base
         include StatementModifier
         include LineLengthHelp
         include IgnoredPattern
+        include RangeHelp
         extend AutoCorrector
 
         MSG_USE_MODIFIER = 'Favor modifier `%<keyword>s` usage when having a ' \
@@ -66,7 +67,16 @@ def on_if(node)
 
         def autocorrect(corrector, node)
           replacement = if node.modifier_form?
-                          to_normal_form(node)
+                          indentation = ' ' * node.source_range.column
+                          last_argument = node.if_branch.last_argument
+
+                          if last_argument.respond_to?(:heredoc?) && last_argument.heredoc?
+                            heredoc = extract_heredoc_from(last_argument)
+                            remove_heredoc(corrector, heredoc)
+                            to_normal_form_with_heredoc(node, indentation, heredoc)
+                          else
+                            to_normal_form(node, indentation)
+                          end
                         else
                           to_modifier_form(node)
                         end
@@ -151,14 +161,38 @@ def another_statement_on_same_line?(node)
           node && (sibling = node.children[index + 1]) && sibling.source_range.first_line == line_no
         end
 
-        def to_normal_form(node)
-          indentation = ' ' * node.source_range.column
+        def to_normal_form(node, indentation)
           <<~RUBY.chomp
             #{node.keyword} #{node.condition.source}
             #{indentation}  #{node.body.source}
             #{indentation}end
           RUBY
         end
+
+        def to_normal_form_with_heredoc(node, indentation, heredoc)
+          heredoc_body, heredoc_end = heredoc
+
+          <<~RUBY.chomp
+            #{node.keyword} #{node.condition.source}
+            #{indentation}  #{node.body.source}
+            #{indentation}  #{heredoc_body.source.chomp}
+            #{indentation}  #{heredoc_end.source.chomp}
+            #{indentation}end
+          RUBY
+        end
+
+        def extract_heredoc_from(last_argument)
+          heredoc_body = last_argument.loc.heredoc_body
+          heredoc_end = last_argument.loc.heredoc_end
+
+          [heredoc_body, heredoc_end]
+        end
+
+        def remove_heredoc(corrector, heredoc)
+          heredoc.each do |range|
+            corrector.remove(range_by_whole_lines(range, include_final_newline: true))
+          end
+        end
       end
     end
   end

spec/rubocop/cop/style/if_unless_modifier_spec.rb

@@ -73,6 +73,25 @@ def f
         end
       end
 
+      context 'when using a method with heredoc argument' do
+        it 'accepts' do
+          expect_offense(<<~RUBY)
+            fooooooooooooooooooooooooooooooooooooooooooooooooooooooooooo(<<~EOS) if condition
+                                                                                 ^^ Modifier form of `if` makes the line too long.
+              string
+            EOS
+          RUBY
+
+          expect_correction(<<~RUBY)
+            if condition
+              fooooooooooooooooooooooooooooooooooooooooooooooooooooooooooo(<<~EOS)
+                string
+              EOS
+            end
+          RUBY
+        end
+      end
+
       describe 'IgnoreCopDirectives' do
         let(:spaces) { ' ' * 57 }
         let(:comment) { '# rubocop:disable Style/For' }