Fix String index is out of bounds crash

Generator/Source/Internal/Tokenizer.swift

@@ -187,7 +187,7 @@ public struct Tokenizer {
 
             let effects: InstanceVariable.Effects
             if let bodyRange = bodyRange {
-                effects = parseEffects(source: source[bodyRange])
+                effects = parseEffects(source: source.utf8[bodyRange])
             } else {
                 effects = .init()
             }

Tests/Swift/Source/TestedClass.swift

@@ -27,6 +27,11 @@ class TestedClass {
         return "a"
     }
 
+    var readOnlyPropertyWithJapaneseComment: String {
+        // 日本語コメント
+        return "a"
+    }
+
     @available(iOS 42.0, *)
     var unavailableProperty: UnavailableProtocol? {
         return nil