Prevent PrintOptions from being implicitly copied.

NFC *except* that I noticed a bug by inspection where we suppress
`@escaping` when print enum element types. Since this affects
recursive positions, we end up suppressing `@escaping` in places
we shouldn't. This is unlikely to affect much real code, but should
still obviously be fixed.

The new design is a little sketchy in that we're using `const` to
prevent direct use (and allow initialization of `const &` parameters)
but still relying on modification of the actual object.  Essentially,
we are treating the `const`-ness of the reference as a promise to leave
the original value in the object after computation rather than a
guarantee of not modifying the object. This is okay --- a temporary
bound to a `const` reference is still a non-`const` object formally
and can be modified without invoking UB --- but makes me a little
uncomfortable.

include/swift/Sema/ConstraintGraph.h

@@ -216,7 +216,7 @@ private:
 
   /// Print this graph node.
   void print(llvm::raw_ostream &out, unsigned indent,
-             PrintOptions PO = PrintOptions()) const;
+             const PrintOptions &PO = PrintOptions()) const;
 
   SWIFT_DEBUG_DUMP;