[CSOptimizer] Consider choices marked as @_disfavoredOverload

These choices could be better than some other non-disfavored ones
in certain situations i.e. when `async` overload is disfavored
but appears in async context it's preferrable to a non-async
overload choice.

Author: xedin

lib/Sema/CSOptimizer.cpp

@@ -371,10 +371,9 @@ static ValueDecl *isViableOverloadChoice(ConstraintSystem &cs,
     }
   }
 
-  // If disjunction choice is unavailable or disfavored we cannot
+  // If disjunction choice is unavailable we cannot
   // do anything with it.
-  if (decl->getAttrs().hasAttribute<DisfavoredOverloadAttr>() ||
-      cs.isDeclUnavailable(decl, locator))
+  if (cs.isDeclUnavailable(decl, locator))
     return nullptr;
 
   return decl;

test/Constraints/old_hack_related_ambiguities.swift

@@ -360,3 +360,14 @@ class TestFailableOnly {
     }
   }
 }
+
+do {
+  @_disfavoredOverload
+  func test(over: Int, that: String = "", block: @escaping (Int) throws -> Void) async throws {}
+  func test(over: Int, that: String = "", block: @escaping (Int) throws -> Void) throws {}
+  func test(over: Int, other: String = "", block: @escaping (Int) throws -> Void) throws {}
+
+  func perform(v: Int, block: @escaping (Int) throws -> Void) async throws {
+    try await test(over: v, block: block) // Ok
+  }
+}