[Tooling] Produce diagnostics for missing input files.

[sammccall]

Summary

This was disabled way back in 2011, in the dark times before Driver was VFS-aware.

Also, make driver more VFS-aware :-)

This breaks one ClangTidy test (we improved the error message), will fix when
submitting.

lib/Driver/Driver.cpp

static bool DiagnoseInputExistence(const Driver &D, const DerivedArgList &Args,
   if (Arg *WorkDir = Args.getLastArg(options::OPT_working_directory)) {
     if (!llvm::sys::path::is_absolute(Path)) {
       SmallString<64> Directory(WorkDir->getValue());
       llvm::sys::path::append(Directory, Value);
       Path.assign(Directory);
     }
   }
 
-  if (llvm::sys::fs::exists(Twine(Path)))
+  if (D.getVFS().exists(Path))
     return true;
 
   if (D.IsCLMode()) {
     if (!llvm::sys::path::is_absolute(Twine(Path)) &&
         llvm::sys::Process::FindInEnvPath("LIB", Value))
       return true;
 
     if (Args.hasArg(options::OPT__SLASH_link) && Ty == types::TY_Object) {

lib/Tooling/Tooling.cpp

bool ToolInvocation::run() {
   TextDiagnosticPrinter DiagnosticPrinter(
       llvm::errs(), &*DiagOpts);
   DiagnosticsEngine Diagnostics(
       IntrusiveRefCntPtr<DiagnosticIDs>(new DiagnosticIDs()), &*DiagOpts,
       DiagConsumer ? DiagConsumer : &DiagnosticPrinter, false);
 
   const std::unique_ptr<driver::Driver> Driver(
       newDriver(&Diagnostics, BinaryName, Files->getVirtualFileSystem()));
-  // Since the input might only be virtual, don't check whether it exists.
-  Driver->setCheckInputsExist(false);
   const std::unique_ptr<driver::Compilation> Compilation(
       Driver->BuildCompilation(llvm::makeArrayRef(Argv)));
   if (!Compilation)
     return false;
   const llvm::opt::ArgStringList *const CC1Args = getCC1Arguments(
       &Diagnostics, Compilation.get());
   if (!CC1Args)
     return false;