Don't visit synthesized extensions twice.

Teach `getTopLevelDeclsWithAuxiliaryDecls` not to provide extension
declarations, because those are covered by the synthesized file, which
all clients need to walk anyway. Without this, we end up asserting in
TBD generation about duplicate symbol visitation.

Encountered while investigating rdar://108056018.

Author: DougGregor

lib/AST/Module.cpp

@@ -4072,7 +4072,8 @@ void FileUnit::getTopLevelDeclsWithAuxiliaryDecls(
   getTopLevelDecls(nonExpandedDecls);
   for (auto *decl : nonExpandedDecls) {
     decl->visitAuxiliaryDecls([&](Decl *auxDecl) {
-      results.push_back(auxDecl);
+      if (!isa<ExtensionDecl>(auxDecl))
+        results.push_back(auxDecl);
     });
     results.push_back(decl);
   }

test/Macros/macro_expand_conformances.swift

@@ -5,7 +5,7 @@
 // RUN: %target-swift-frontend -swift-version 5 -typecheck -load-plugin-library %t/%target-library-name(MacroDefinition) %s -disable-availability-checking -dump-macro-expansions > %t/expansions-dump.txt 2>&1
 // RUN: %FileCheck -check-prefix=CHECK-DUMP %s < %t/expansions-dump.txt
 // RUN: %target-typecheck-verify-swift -swift-version 5 -load-plugin-library %t/%target-library-name(MacroDefinition) -module-name MacroUser -DTEST_DIAGNOSTICS -swift-version 5
-// RUN: %target-build-swift -swift-version 5 -load-plugin-library %t/%target-library-name(MacroDefinition) %s -o %t/main -module-name MacroUser -swift-version 5
+// RUN: %target-build-swift -swift-version 5 -load-plugin-library %t/%target-library-name(MacroDefinition) %s -o %t/main -module-name MacroUser -swift-version 5 -emit-tbd -emit-tbd-path %t/MacroUser.tbd
 // RUN: %target-codesign %t/main
 // RUN: %target-run %t/main | %FileCheck %s