Revert "[ConstraintSystem] Extend availability check to cover unavailable extensions."

This reverts commit 6de6de2.

Author: tshortli

lib/Sema/ConstraintSystem.cpp

@@ -6540,7 +6540,7 @@ void ConstraintSystem::diagnoseFailureFor(SolutionApplicationTarget target) {
 bool ConstraintSystem::isDeclUnavailable(const Decl *D,
                                          ConstraintLocator *locator) const {
   // First check whether this declaration is universally unavailable.
-  if (AvailableAttr::isUnavailable(D))
+  if (D->getAttrs().isUnavailable(getASTContext()))
     return true;
 
   return TypeChecker::isDeclarationUnavailable(D, DC, [&] {

test/Constraints/availability.swift

@@ -43,25 +43,3 @@ func unavailableFunction(_ x: Int) -> Bool { true } // expected-note {{'unavaila
 func f_55700(_ arr: [Int]) {
   for x in arr where unavailableFunction(x) {} // expected-error {{'unavailableFunction' is unavailable}}
 }
-
-// rdar://92364955 - ambiguity with member declared in unavailable extension
- struct WithUnavailableExt {
- }
-
- @available(*, unavailable)
- extension WithUnavailableExt {
-   static var foo: WithUnavailableExt = WithUnavailableExt()
- }
-
- func test_no_ambiguity_with_unavailable_ext() {
-   struct A {
-     static var foo: A = A()
-   }
-
-   struct Test {
-     init(_: A) {}
-     init(_: WithUnavailableExt) {}
-   }
-
-   _ = Test(.foo) // Ok `A.foo` since `foo` from `WithUnavailableExt` is unavailable
- }