ModuleInterface: Stop printing $CustomAvailability guards in swiftinterfaces.

All supported compilers can now parse `@available` attributes that reference
custom availability domains.

Author: tshortli

lib/AST/FeatureSet.cpp

@@ -354,14 +354,7 @@ static bool usesFeatureCoroutineAccessors(Decl *decl) {
 }
 
 UNINTERESTING_FEATURE(GeneralizedIsSameMetaTypeBuiltin)
-
-static bool usesFeatureCustomAvailability(Decl *decl) {
-  for (auto attr : decl->getSemanticAvailableAttrs()) {
-    if (attr.getDomain().isCustom())
-      return true;
-  }
-  return false;
-}
+UNINTERESTING_FEATURE(CustomAvailability)
 
 static bool usesFeatureAsyncExecutionBehaviorAttributes(Decl *decl) {
   // Explicit `@concurrent` attribute on the declaration.

test/ModuleInterface/availability_custom_domains.swift

@@ -15,18 +15,16 @@
 
 import Oceans // re-exports Rivers
 
-// CHECK:      #if compiler(>=5.3) && $CustomAvailability
-// CHECK-NEXT: @available(Colorado)
+// CHECK-NOT:  $CustomAvailability
+
+// CHECK:      @available(Colorado)
 // CHECK-NEXT: public func availableInColorado()
-// CHECK-NEXT: #endif
 @available(Colorado)
 public func availableInColorado() { }
 
-// CHECK:      #if compiler(>=5.3) && $CustomAvailability
-// CHECK-NEXT: @available(Arctic, unavailable)
+// CHECK:      @available(Arctic, unavailable)
 // CHECK-NEXT: @available(Pacific)
 // CHECK-NEXT: public func unavailableInArcticButAvailableInPacific()
-// CHECK-NEXT: #endif
 @available(Arctic, unavailable)
 @available(Pacific)
 public func unavailableInArcticButAvailableInPacific() { }