protected static void addEscapedChar(HashMap<Character, String> map, char key) {
		addEscapedChar(map, key, key);
	}
For pure strings of Unicode char, how can we display
 it in the target language as a literal. Useful for dumping
 predicates and such that may refer to chars that need to be escaped
 when represented as strings.  Also, templates need to be escaped so
 that the target language can hold them as a string.
 Each target can have a different set in memory at same time.
tool/src/org/antlr/v4/codegen/Target.java