Fix interpolated heredoc

This fixes https://bugs.ruby-lang.org/issues/18038. The provided
reproduction showed that this happens in heredocs with double
interpolation. In this case `DSTR` was getting returned but needs to be
convered to a `EVSTR` which is what is returned by the function. There
may be an additional bug here that we weren't able to produce. It seems
odd that `STR` returns `DSTR` while everything else should return
`EVSTR` since the function is `new_evstr`.

[Bug #18038][ruby-core:104597]

Co-authored-by: Aaron Patterson <tenderlove@ruby-lang.org>

parse.y

@@ -10181,7 +10181,10 @@ new_evstr(struct parser_params *p, NODE *node, const YYLTYPE *loc)
 	switch (nd_type(node)) {
 	  case NODE_STR:
 	    nd_set_type(node, NODE_DSTR);
-	  case NODE_DSTR: case NODE_EVSTR:
+            return node;
+          case NODE_DSTR:
+            break;
+          case NODE_EVSTR:
 	    return node;
 	}
     }

test/ruby/test_parse.rb

@@ -1080,6 +1080,32 @@ x = __ENCODING__
     end;
   end
 
+    def test_heredoc_interpolation
+      var = 1
+
+      v1 = <<~HEREDOC
+        something
+        #{"/#{var}"}
+      HEREDOC
+
+      v2 = <<~HEREDOC
+        something
+        #{other = "/#{var}"}
+      HEREDOC
+
+      v3 = <<~HEREDOC
+        something
+        #{("/#{var}")}
+      HEREDOC
+
+      assert_equal "something\n/1\n", v1
+      assert_equal "something\n/1\n", v2
+      assert_equal "something\n/1\n", v3
+      assert_equal v1, v2
+      assert_equal v2, v3
+      assert_equal v1, v3
+    end
+
   def test_unexpected_token_error
     assert_syntax_error('"x"xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx', /unexpected/)
   end