[Frontend] Re-enable imported AST verification

The reasons for disabling it appear to now be
outdated. Re-enable it, and run it after the
entire pipeline has finished to make sure we
verify any decls deserialised during SIL/IRGen.

Author: hamishknight

lib/FrontendTool/FrontendTool.cpp

@@ -1209,6 +1209,9 @@ static bool emitAnyWholeModulePostTypeCheckSupplementaryOutputs(
 static void performEndOfPipelineActions(CompilerInstance &Instance) {
   assert(Instance.hasASTContext());
 
+  // Verify the AST for all the modules we've loaded.
+  Instance.getASTContext().verifyAllLoadedModules();
+
   // Emit dependencies and index data.
   emitReferenceDependenciesForAllPrimaryInputsIfNeeded(Instance);
   emitIndexData(Instance);

lib/Sema/TypeChecker.cpp

@@ -379,20 +379,6 @@ void swift::performWholeModuleTypeChecking(SourceFile &SF) {
   diagnoseObjCMethodConflicts(SF);
   diagnoseObjCUnsatisfiedOptReqConflicts(SF);
   diagnoseUnintendedObjCMethodOverrides(SF);
-
-  // In whole-module mode, import verification is deferred until all files have
-  // been type checked. This avoids caching imported declarations when a valid
-  // type checker is not present. The same declaration may need to be fully
-  // imported by subsequent files.
-  //
-  // FIXME: some playgrounds tests (playground_lvalues.swift) fail with
-  // verification enabled.
-#if 0
-  if (SF.Kind != SourceFileKind::SIL &&
-      !Ctx.LangOpts.DebuggerSupport) {
-    Ctx.verifyAllLoadedModules();
-  }
-#endif
 }
 
 bool swift::isAdditiveArithmeticConformanceDerivationEnabled(SourceFile &SF) {