[Macros] Treat MacroDecl as a local context, like functions are.

Author: DougGregor

include/swift/AST/DeclContext.h

@@ -94,14 +94,14 @@ enum class DeclContextKind : unsigned {
   EnumElementDecl,
   AbstractFunctionDecl,
   SerializedLocal,
-  Last_LocalDeclContextKind = SerializedLocal,
+  MacroDecl,
+  Last_LocalDeclContextKind = MacroDecl,
 
   Module,
   FileUnit,
   GenericTypeDecl,
   ExtensionDecl,
-  MacroDecl,
-  Last_DeclContextKind = MacroDecl
+  Last_DeclContextKind = ExtensionDecl
 };
 
 /// Kinds of DeclContexts after deserialization.

lib/AST/ASTMangler.cpp

@@ -2381,7 +2381,7 @@ void ASTMangler::appendContext(const DeclContext *ctx, StringRef useModuleName)
     return appendContext(ctx->getParent(), useModuleName);
 
   case DeclContextKind::MacroDecl:
-    llvm_unreachable("macro declarations are never mangled");
+    return appendContext(ctx->getParent(), useModuleName);
   }
 
   llvm_unreachable("bad decl context");

lib/Serialization/Serialization.cpp

@@ -485,7 +485,8 @@ static ModuleDecl *getModule(ModuleOrSourceFile DC) {
 
 static bool shouldSerializeAsLocalContext(const DeclContext *DC) {
   return DC->isLocalContext() && !isa<AbstractFunctionDecl>(DC) &&
-        !isa<SubscriptDecl>(DC) && !isa<EnumElementDecl>(DC);
+        !isa<SubscriptDecl>(DC) && !isa<EnumElementDecl>(DC) &&
+        !isa<MacroDecl>(DC);
 }
 
 namespace {