ASTPrinter: Fix printing of let properties inside @objc @implementation extensions

A `let` here is really just a `var` with a `{ get }`.

Fixes rdar://problem/127120469.

test/ModuleInterface/Inputs/objc_implementation/objc_implementation.h

@@ -4,6 +4,7 @@
 
 - (nonnull instancetype)init;
 
+@property (readonly) int letProperty1;
 @property (assign) int implProperty;
 
 - (void)mainMethod:(int)param;