[Macros] Don't visit auxiliary decls of class members twice.

lib/Sema/TypeCheckDeclPrimary.cpp

@@ -1821,9 +1821,13 @@ class DeclChecker : public DeclVisitor<DeclChecker> {
 
   void visit(Decl *decl) {
     // Visit auxiliary decls first.
-    decl->visitAuxiliaryDecls([&](Decl *auxiliaryDecl) {
-      this->visit(auxiliaryDecl);
-    });
+    // We don't do this for members of classes because it happens as part of
+    // visiting their ABI members.
+    if (!isa<ClassDecl>(decl->getDeclContext())) {
+      decl->visitAuxiliaryDecls([&](Decl *auxiliaryDecl) {
+        this->visit(auxiliaryDecl);
+      });
+    }
 
     if (auto *Stats = getASTContext().Stats)
       ++Stats->getFrontendCounters().NumDeclsTypechecked;

test/Macros/macro_expand_peers.swift

@@ -68,6 +68,14 @@ func useCompletionHandlerG(s: S, _ body: @escaping (String) -> Void) {
   }
 }
 
+class C {
+  @addCompletionHandler
+  func f(a: Int, for b: String, _ value: Double) async -> String {
+    return b
+  }
+}
+
+
 @addCompletionHandler
 func f(a: Int, for b: String, _ value: Double) async -> String {
   return b