CustomAvailability: avoid exposing the synthesized function for dynamic availability checking as an API entry

For dynamic features defined from a header, we synthesize a pure Clang function to check whether the feature should
be enabled at runtime. Swift modules don't have capability to deserialize this clang predicate function, leading to
crasher as a result. This change fixes the crasher by hiding the synthesized function to be a module internal one.

rdar://164410957

test/Availability/Inputs/AvailabilityDomains.h

@@ -0,0 +1,33 @@
+#ifndef __AVAILABILITY_DOMAIN_TEST_H
+#define __AVAILABILITY_DOMAIN_TEST_H
+
+#include <availability_domain.h>
+
+int dynamic_domain_pred();
+
+CLANG_ENABLED_AVAILABILITY_DOMAIN(EnabledDomain);
+CLANG_DISABLED_AVAILABILITY_DOMAIN(DisabledDomain);
+CLANG_DYNAMIC_AVAILABILITY_DOMAIN(DynamicDomain, dynamic_domain_pred);
+
+#define AVAIL 0
+#define UNAVAIL 1
+
+__attribute__((availability(domain : EnabledDomain, AVAIL))) void
+available_in_enabled_domain(void);
+
+__attribute__((availability(domain : EnabledDomain, UNAVAIL))) void
+unavailable_in_enabled_domain(void);
+
+__attribute__((availability(domain : DisabledDomain, AVAIL))) void
+available_in_disabled_domain(void);
+
+__attribute__((availability(domain : DisabledDomain, UNAVAIL))) void
+unavailable_in_disabled_domain(void);
+
+__attribute__((availability(domain : DynamicDomain, AVAIL))) void
+available_in_dynamic_domain(void);
+
+__attribute__((availability(domain : DynamicDomain, UNAVAIL))) void
+unavailable_in_dynamic_domain(void);
+
+#endif // __AVAILABILITY_DOMAIN_TEST_H

test/Availability/availability_custom_domains_clang_serialization.swift

@@ -0,0 +1,43 @@
+// REQUIRES: swift_feature_CustomAvailability
+// RUN: %empty-directory(%t/mod)
+// RUN: %target-swift-frontend -emit-module %s -o %t/mod/Foo.swiftmodule -parse-as-library -enable-library-evolution -module-name Foo -import-bridging-header %S/Inputs/AvailabilityDomains.h -enable-experimental-feature CustomAvailability -D FOO
+// RUN: %target-swift-frontend -emit-module %s -o %t/mod/Bar.swiftmodule -parse-as-library -enable-library-evolution -module-name Bar -import-bridging-header %S/Inputs/AvailabilityDomains.h -enable-experimental-feature CustomAvailability -I %t/mod -D BAR
+
+#if FOO
+
+@available(DynamicDomain)
+public struct X {
+    public init() { }
+}
+
+public struct Z {
+    public init() {
+        if #available(DynamicDomain) {
+            print("#available")
+            print(X())
+        } else {
+            print("#unavailable")
+        }
+    }
+}
+
+#endif
+
+#if BAR
+
+import Foo
+
+public struct Y {
+    init() {
+        if #available(DynamicDomain) {
+            print("#available")
+            print(X())
+            print(Z())
+        } else {
+            print("#unavailable")
+            print(Z())
+        }
+    }
+
+}
+#endif