[frontend] Fix use-after-free issue found via ASAN

(cherry picked from commit e6f3af8)

Author: akyrtzi

lib/Frontend/Frontend.cpp

@@ -638,4 +638,7 @@ void CompilerInstance::performParseOnly() {
 void CompilerInstance::freeContextAndSIL() {
   Context.reset();
   TheSILModule.reset();
+  MainModule = nullptr;
+  SML = nullptr;
+  PrimarySourceFile = nullptr;
 }

lib/FrontendTool/FrontendTool.cpp

@@ -910,10 +910,9 @@ static bool performCompile(CompilerInstance &Instance,
   // Free up some compiler resources now that we have an IRModule.
   Instance.freeContextAndSIL();
 
-  DiagnosticEngine &Diags = Context.Diags;
   // Now that we have a single IR Module, hand it over to performLLVM.
-  return performLLVM(IRGenOpts, &Diags, nullptr, HashGlobal, IRModule.get(),
-                  TargetMachine.get(), EffectiveLanguageVersion,
+  return performLLVM(IRGenOpts, &Instance.getDiags(), nullptr, HashGlobal,
+                  IRModule.get(), TargetMachine.get(), EffectiveLanguageVersion,
                   opts.getSingleOutputFilename()) || HadError;
 }