public static int getCharValueFromGrammarCharLiteral(String literal) {
		if ( literal==null || literal.length()<3 ) return -1;
		return getCharValueFromCharInGrammarLiteral(literal.substring(1,literal.length()-1));
	}
Given a literal like (the 3 char sequence with single quotes) 'a',
 return the int value of 'a'. Convert escape sequences here also.
 Return -1 if not single char.
tool/src/org/antlr/v4/misc/CharSupport.java