AST: Import "always enabled" availability domains from Clang.

Resolves rdar://157593409.

Author: tshortli

lib/AST/AvailabilityDomain.cpp

@@ -33,7 +33,8 @@ getCustomDomainKind(clang::FeatureAvailKind featureAvailKind) {
     return CustomAvailabilityDomain::Kind::Disabled;
   case clang::FeatureAvailKind::Dynamic:
     return CustomAvailabilityDomain::Kind::Dynamic;
-  // FIXME: [availability] Add support for AlwaysEnabled.
+  case clang::FeatureAvailKind::AlwaysAvailable:
+    return CustomAvailabilityDomain::Kind::AlwaysEnabled;
   default:
     llvm::report_fatal_error("unexpected kind");
   }
@@ -54,11 +55,11 @@ customDomainForClangDecl(ValueDecl *decl) {
     return nullptr;
 
   // Check that the domain has a supported availability kind.
-  // FIXME: [availability] Add support for AlwaysEnabled.
   switch (featureInfo.second.Kind) {
   case clang::FeatureAvailKind::Available:
   case clang::FeatureAvailKind::Unavailable:
   case clang::FeatureAvailKind::Dynamic:
+  case clang::FeatureAvailKind::AlwaysAvailable:
     break;
   default:
     return nullptr;

test/ClangImporter/Inputs/availability_custom_domains_other.swift

@@ -9,4 +9,26 @@ func availableInMediterranean() { }
 func testOtherClangDecls() { // expected-note {{add '@available' attribute to enclosing global function}}
   available_in_baltic() // expected-error {{'available_in_baltic()' is only available in Baltic}}
   // expected-note@-1 {{add 'if #available' version check}}
+  available_in_bering() // ok, Bering is always available
+  unavailable_in_bering() // expected-error {{'unavailable_in_bering()' is unavailable}}
+}
+
+@available(Baltic)
+func availableInBalticOther() {
+  available_in_baltic()
+  available_in_bering() // ok, Bering is always available
+  unavailable_in_bering() // expected-error {{'unavailable_in_bering()' is unavailable}}
+}
+
+@available(Bering)
+func availableInBering() { // expected-note {{add '@available' attribute to enclosing global function}}
+  available_in_baltic() // expected-error {{'available_in_baltic()' is only available in Baltic}}
+  // expected-note@-1 {{add 'if #available' version check}}
+  available_in_bering()
+  unavailable_in_bering() // expected-error {{'unavailable_in_bering()' is unavailable}}
+}
+
+@available(Bering, unavailable)
+func unavailableInBering() {
+  unavailable_in_bering()
 }

test/Inputs/custom-modules/availability-domains/Seas.h

@@ -3,6 +3,7 @@
 int aegean_pred(void);
 
 CLANG_ENABLED_AVAILABILITY_DOMAIN(Baltic);
+CLANG_ALWAYS_ENABLED_AVAILABILITY_DOMAIN(Bering);
 CLANG_DISABLED_AVAILABILITY_DOMAIN(Mediterranean);
 CLANG_DYNAMIC_AVAILABILITY_DOMAIN(Aegean, aegean_pred);
 
@@ -12,5 +13,11 @@ CLANG_DYNAMIC_AVAILABILITY_DOMAIN(Aegean, aegean_pred);
 __attribute__((availability(domain:Baltic, AVAIL)))
 void available_in_baltic(void);
 
+__attribute__((availability(domain:Bering, AVAIL)))
+void available_in_bering(void);
+
+__attribute__((availability(domain:Bering, UNAVAIL)))
+void unavailable_in_bering(void);
+
 #undef UNAVAIL
 #undef AVAIL