AST: Handle MacroExpansionDecl in DeclExportabilityVisitor.

Instances of `MacroExpansionDecl` may be found in the AST when declarations are
expanded from freestanding declaration macros and therefore exportability
computations must be prepared to handle them.

Resolves rdar://119449439

Author: tshortli

include/swift/AST/DeclExportabilityVisitor.h

@@ -153,7 +153,6 @@ class DeclExportabilityVisitor
   UNREACHABLE(PoundDiagnostic);
   UNREACHABLE(Missing);
   UNREACHABLE(MissingMember);
-  UNREACHABLE(MacroExpansion);
   UNREACHABLE(GenericTypeParam);
   UNREACHABLE(Param);
 
@@ -172,6 +171,7 @@ class DeclExportabilityVisitor
   UNINTERESTING(PrecedenceGroup);
   UNINTERESTING(EnumCase);
   UNINTERESTING(Operator);
+  UNINTERESTING(MacroExpansion);
 
 #undef UNINTERESTING
 };

test/Macros/skip_non_exportable_decls.swift

@@ -0,0 +1,17 @@
+// REQUIRES: swift_swift_parser, executable_test
+
+// RUN: %empty-directory(%t)
+// RUN: %host-build-swift -swift-version 5 -emit-library -o %t/%target-library-name(MacroDefinition) -module-name=MacroDefinition %S/Inputs/syntax_macro_definitions.swift
+
+// RUN: %target-swift-frontend -swift-version 5 -emit-module -o %t/freestanding_macro_library.swiftmodule %S/Inputs/freestanding_macro_library.swift -module-name freestanding_macro_library -load-plugin-library %t/%target-library-name(MacroDefinition)
+
+// RUN: %target-swift-frontend -parse-as-library -emit-sil -load-plugin-library %t/%target-library-name(MacroDefinition) %s -module-name MacroUser -experimental-skip-non-exportable-decls | %FileCheck %s
+
+@freestanding(declaration)
+macro anonymousTypes(public: Bool = false, causeErrors: Bool = false, _: () -> String) = #externalMacro(module: "MacroDefinition", type: "DefineAnonymousTypesMacro")
+
+// CHECK: sil @$s9MacroUser03$s9A70User33_B2D49A1BE4DC7AF5CC327EB8EE2214BDLl14anonymousTypesfMf_4namefMu_C5helloSSyF : $@convention(method) (@guaranteed $s9MacroUser33_B2D49A1BE4DC7AF5CC327EB8EE2214BDLl14anonymousTypesfMf_4namefMu_) -> @owned String {
+#anonymousTypes(public: true) { "hello" }
+
+// CHECK-NOT: s9MacroUser03$s9A71User33_B2D49A1BE4DC7AF5CC327EB8EE2214BDLl14anonymousTypesfMf_4namefMu0_O5helloSSyF
+#anonymousTypes(public: false) { "goodbye" }