ClangImporter: Update Swift for changes in clang (PCH + Modules issues)

This sets the NeededByPCHOrCompilationUsesPCH language option when
emitting a brigding PCH.

rdar://problem/35056912
rdar://problem/30384801

Initial patch by Adrian Prantl

Author: bcardosolopes

lib/ClangImporter/ClangImporter.cpp

@@ -983,6 +983,7 @@ ClangImporter::create(ASTContext &ctx,
   if (importerOpts.Mode == ClangImporterOptions::Modes::EmbedBitcode)
     return importer;
 
+  instance.getLangOpts().NeededByPCHOrCompilationUsesPCH = true;
   bool canBegin = action->BeginSourceFile(instance,
                                           instance.getFrontendOpts().Inputs[0]);
   if (!canBegin)
@@ -1361,6 +1362,7 @@ ClangImporter::emitBridgingPCH(StringRef headerPath,
   invocation->getFrontendOpts().OutputFile = outputPCHPath;
   invocation->getFrontendOpts().ProgramAction = clang::frontend::GeneratePCH;
   invocation->getPreprocessorOpts().resetNonModularOptions();
+  invocation->getLangOpts()->NeededByPCHOrCompilationUsesPCH = true;
 
   clang::CompilerInstance emitInstance(
     Impl.Instance->getPCHContainerOperations());