Add PM_STRING_FLAGS_FROZEN / PM_STRING_FLAGS_MUTABLE on PM_SOURCE_FILE_NODE

For all intent and purposes, `__FILE__` is a string literal subject
to the `# frozen_string_literal: true/false` comment and to the
global `--enable-frozen-string-literal / --disable-frozen-string-literal`
CLI flags.

Author: byroot

config.yml

@@ -3090,6 +3090,9 @@ nodes:
           ^^^^^^^^^^^^
   - name: SourceFileNode
     fields:
+      - name: flags
+        type: flags
+        kind: StringFlags
       - name: filepath
         type: string
     comment: |

src/prism.c

@@ -5782,10 +5782,21 @@ pm_source_file_node_create(pm_parser_t *parser, const pm_token_t *file_keyword)
     pm_source_file_node_t *node = PM_ALLOC_NODE(parser, pm_source_file_node_t);
     assert(file_keyword->type == PM_TOKEN_KEYWORD___FILE__);
 
+    pm_node_flags_t flags = 0;
+
+    switch (parser->frozen_string_literal) {
+        case PM_OPTIONS_FROZEN_STRING_LITERAL_DISABLED:
+            flags |= PM_STRING_FLAGS_MUTABLE;
+            break;
+        case PM_OPTIONS_FROZEN_STRING_LITERAL_ENABLED:
+            flags |= PM_NODE_FLAG_STATIC_LITERAL | PM_STRING_FLAGS_FROZEN;
+            break;
+    }
+
     *node = (pm_source_file_node_t) {
         {
             .type = PM_SOURCE_FILE_NODE,
-            .flags = PM_NODE_FLAG_STATIC_LITERAL,
+            .flags = flags,
             .location = PM_LOCATION_TOKEN_VALUE(file_keyword),
         },
         .filepath = parser->filepath

test/prism/snapshots/keyword_method_names.txt

@@ -58,7 +58,7 @@ def test_source_encoding
     end
 
     def test_source_file
-      assert_equal __FILE__.inspect, static_inspect("__FILE__", filepath: __FILE__)
+      assert_equal __FILE__.inspect, static_inspect("__FILE__", filepath: __FILE__, frozen_string_literal: true)
     end
 
     def test_source_line

test/prism/snapshots/keywords.txt

@@ -34,7 +34,6 @@ def test_static_literals
 
       assert_warning("\"#{__FILE__}\"")
       assert_warning("\"foo\"")
-      assert_warning("\"#{__FILE__}\"", "__FILE__")
 
       assert_warning("/foo/")