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

lib/ClangImporter/SwiftDeclSynthesizer.cpp

@@ -3183,9 +3183,7 @@ FuncDecl *SwiftDeclSynthesizer::makeAvailabilityDomainPredicate(
       BuiltinIntegerType::get(1, ctx), ImporterImpl.ImportedHeaderUnit);
   funcDecl->setBodySynthesizer(synthesizeAvailabilityDomainPredicateBody,
                                (void *)var);
-  funcDecl->setAccess(AccessLevel::Public);
-  funcDecl->addAttribute(
-      new (ctx) ExportAttr(ExportKind::Implementation, /*IsImplicit=*/true));
+  funcDecl->setAccess(AccessLevel::Internal);
 
   ImporterImpl.availabilityDomainPredicates[var] = funcDecl;