[ParseableInterfaces] Re-escape multi-line strings in attribute messages

Previously, we would print multi-line string literals with single quotes, which were not re-parseable. Instead, re-escape their contents and print them out escaped.

include/swift/AST/ASTPrinter.h

@@ -207,6 +207,8 @@ public:
     return *this;
   }
 
+  void printEscapedStringLiteral(StringRef str);
+
   void printName(Identifier Name,
                  PrintNameContext Context = PrintNameContext::Normal);