Flags for interpolated string nodes that indicated mutability if they are also marked as literals.
Constants
frozen by virtue of a ‘frozen_string_literal: true` comment or --enable-frozen-string-literal; only for adjacent string literals like `’a’ ‘b’‘
mutable by virtue of a ‘frozen_string_literal: false` comment or --disable-frozen-string-literal; only for adjacent string literals like `’a’ ‘b’‘