Character literal escapes not preserved in generated stubs

[rsmckinney]

An extension class on the following type will fail.

public class StringTag implements Tag {
    . . .
    private static final char SINGLE_QUOTE = '\'';
    . . .
}

.../StringTag.java:11: error: empty character literal
    private static final char SINGLE_QUOTE = (char)''';
                                                   ^
.../StringTag.java:11: error: unclosed character literal
    private static final char SINGLE_QUOTE = (char)''';
                                                     ^
An exception has occurred in the compiler (21.0.2). Please file a bug...

From discussion: https://discord.com/channels/1263152297190359132/1377344023227928597