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;

test/ModuleInterface/objc_implementation.swift

@@ -42,6 +42,12 @@ import Foundation
     didSet { print(implProperty) }
   }
 
+  // CHECK-NOT: var letProperty1:
+  @objc public let letProperty1: Int32
+
+  // CHECK-DAG: @nonobjc public var letProperty2: Swift.Int32 { get }
+  @nonobjc public let letProperty2: Int32
+
   // CHECK-DAG: final public var implProperty2: ObjectiveC.NSObject? { get set }
   public final var implProperty2: NSObject?