Clear flags on interpolated strings

When inner strings aren't frozen, we need to clear the flags on
interpolated string nodes so that we don't emit wrong instructions.

The compiler is currently incorrectly emitting frozen strings because
the parser is erroneously declaring interpolated strings as "frozen".
We need to fix this behavior in the parser so we can fix the compiler in
CRuby.  This patch is a partial fix for [this bug](https://bugs.ruby-lang.org/issues/21187)

Author: tenderlove

src/prism.c

@@ -5279,6 +5279,10 @@ pm_interpolated_string_node_append(pm_interpolated_string_node_t *node, pm_node_
 
     switch (PM_NODE_TYPE(part)) {
         case PM_STRING_NODE:
+            // If inner string is not frozen, clear flags for this string
+            if (!PM_NODE_FLAG_P(part, PM_STRING_FLAGS_FROZEN)) {
+                CLEAR_FLAGS(node);
+            }
             part->flags = (pm_node_flags_t) ((part->flags | PM_NODE_FLAG_STATIC_LITERAL | PM_STRING_FLAGS_FROZEN) & ~PM_STRING_FLAGS_MUTABLE);
             break;
         case PM_INTERPOLATED_STRING_NODE:

test/prism/result/static_literals_test.rb

@@ -4,6 +4,11 @@
 
 module Prism
   class StaticLiteralsTest < TestCase
+    def test_concatenanted_string_literal_is_not_static
+      node = Prism.parse_statement("'a' 'b'")
+      refute_predicate node, :static_literal?
+    end
+
     def test_static_literals
       assert_warning("1")
       assert_warning("0xA", "10", "10")

test/prism/result/string_test.rb

@@ -28,5 +28,10 @@ def test_xstring_node_unescaped_frozen
       node = Prism.parse_statement("`foo`")
       assert_predicate node.unescaped, :frozen?
     end
+
+    def test_string_literal_concatenation
+      node = Prism.parse_statement("`foo`")
+      assert_predicate node.unescaped, :frozen?
+    end
   end
 end