CustomAvailability: synthesized dynamic availability checking function should be private

When compiling a Swift module in incremental mode, each Swift source file is compiled into an object file
and we use linker to link them together. Because the predicate function for checking dynamic feature
availability is eagerly synthesized per compilation unit, the linker will complain about duplicated
symbols for them. Setting their visibility as private ensures that linker doesn't see them, thus addressing
the linker errors.

One workaround for this problem is to enable WMO.

rdar://164971313

lib/ClangImporter/SwiftDeclSynthesizer.cpp

@@ -3183,7 +3183,7 @@ FuncDecl *SwiftDeclSynthesizer::makeAvailabilityDomainPredicate(
       BuiltinIntegerType::get(1, ctx), ImporterImpl.ImportedHeaderUnit);
   funcDecl->setBodySynthesizer(synthesizeAvailabilityDomainPredicateBody,
                                (void *)var);
-  funcDecl->setAccess(AccessLevel::Internal);
+  funcDecl->setAccess(AccessLevel::Private);
 
   ImporterImpl.availabilityDomainPredicates[var] = funcDecl;
 

test/Availability/Inputs/AvailabilityDomains.c

@@ -0,0 +1,5 @@
+#include "AvailabilityDomains.h"
+
+int dynamic_domain_pred() {
+	return 1;
+}

test/Availability/Inputs/custom_availability_file1.swift

@@ -0,0 +1,15 @@
+@available(DynamicDomain)
+public struct X {
+    public init() { }
+}
+
+public struct Z {
+    public init() {
+        if #available(DynamicDomain) {
+            print("#available")
+            print(X())
+        } else {
+            print("#unavailable")
+        }
+    }
+}

test/Availability/Inputs/custom_availability_file2.swift

@@ -0,0 +1,12 @@
+public struct Y {
+    init() {
+        if #available(DynamicDomain) {
+            print("#available")
+            print(X())
+            print(Z())
+        } else {
+            print("#unavailable")
+            print(Z())
+        }
+    }
+}

test/Availability/availability_custom_domains_clang_build.swift

@@ -0,0 +1,6 @@
+// REQUIRES: swift_feature_CustomAvailability
+
+// RUN: %empty-directory(%t)
+// RUN: %target-clang -x c %S/Inputs/AvailabilityDomains.c -o %t/AvailabilityDomains.c.o -c
+// RUN: %target-build-swift -emit-library %S/Inputs/custom_availability_file1.swift %S/Inputs/custom_availability_file2.swift -module-name main -enable-experimental-feature CustomAvailability -import-bridging-header %S/Inputs/AvailabilityDomains.h -Xlinker %t/AvailabilityDomains.c.o
+// RUN: %target-build-swift -wmo -emit-library %S/Inputs/custom_availability_file1.swift %S/Inputs/custom_availability_file2.swift -module-name main -enable-experimental-feature CustomAvailability -import-bridging-header %S/Inputs/AvailabilityDomains.h -Xlinker %t/AvailabilityDomains.c.o

test/SILGen/availability_query_custom_domains_clang.swift

@@ -101,7 +101,7 @@ public func testIfAvailableDynamicDomain() {
 }
 // CHECK: end sil function '$s4Test28testIfAvailableDynamicDomainyyF'
 
-// CHECK-LABEL: sil hidden [ossa] @$sSC33__swift_DynamicDomain_isAvailableBi1_yF : $@convention(thin) () -> Builtin.Int1
+// CHECK-LABEL: sil private [ossa] @$sSC33__swift_DynamicDomain_isAvailableBi1_yF : $@convention(thin) () -> Builtin.Int1
 // CHECK: bb0:
 // CHECK:   [[QUERY_FUNC:%.*]] = function_ref @$sSo27__DynamicDomain_isAvailableSbyFTo : $@convention(c) () -> Bool
 // CHECK:   [[QUERY_RESULT:%.*]] = apply [[QUERY_FUNC]]() : $@convention(c) () -> Bool