[Macros] Ensure that name lookup can find uniquely-generated macro names

While a Swift program cannot directly utter one of the unique names
produced by the macro expansion context outside of the macro itself,
utilities such as type reconstruction require the ability to look up
these names. Note that various kinds of macros can introduce unique
names, and update the name lookup facilities for macro-generated names
to account for them.

Fixes rdar://106053984.

lib/Serialization/Serialization.cpp

@@ -6137,7 +6137,8 @@ void Serializer::writeAST(ModuleOrSourceFile DC) {
 
     for (auto D : fileDecls) {
       if (isa<ImportDecl>(D) || isa<IfConfigDecl>(D) ||
-          isa<PoundDiagnosticDecl>(D) || isa<TopLevelCodeDecl>(D)) {
+          isa<PoundDiagnosticDecl>(D) || isa<TopLevelCodeDecl>(D) ||
+          isa<MacroExpansionDecl>(D)) {
         continue;
       }