[CUDA] Raise an error if the CUDA install can't be found.

[jlebar]

Summary

Without this change, we silently proceed on without including
clang_cuda_runtime_wrapper.h. This leads to very strange behavior --
you say you're compiling CUDA code, but e.g. device__ is not defined!

[tra]

LGTM.

include/clang/Basic/DiagnosticDriverKinds.td

 def err_drv_unknown_stdin_type : Error<
   "-E or -x required when input is from standard input">;
 def err_drv_unknown_stdin_type_clang_cl : Error<
   "use /Tc or /Tp to set input type for standard input">;
 def err_drv_unknown_language : Error<"language not recognized: '%0'">;
 def err_drv_invalid_arch_name : Error<
   "invalid arch name '%0'">;
 def err_drv_cuda_bad_gpu_arch : Error<"Unsupported CUDA gpu architecture: %0">;
+def err_drv_no_cuda_installation : Error<
+  "cannot find CUDA installation.  Provide its path via --cuda-path, or pass "
+  "-nocudainc to build without CUDA includes.">;
 def err_drv_invalid_thread_model_for_target : Error<
   "invalid thread model '%0' in '%1' for this target">;
 def err_drv_invalid_linker_name : Error<
   "invalid linker name in argument '%0'">;
 def err_drv_invalid_pgo_instrumentor : Error<
   "invalid PGO instrumentor in argument '%0'">;
 def err_drv_invalid_rtlib_name : Error<
   "invalid runtime library name in argument '%0'">;

lib/Driver/ToolChains.cpp

void Linux::AddClangCXXStdlibIncludeArgs(const ArgList &DriverArgs,
   }
 }
 
 void Linux::AddCudaIncludeArgs(const ArgList &DriverArgs,
                                ArgStringList &CC1Args) const {
   if (DriverArgs.hasArg(options::OPT_nocudainc))
     return;
 
-  if (CudaInstallation.isValid()) {
-    addSystemInclude(DriverArgs, CC1Args, CudaInstallation.getIncludePath());
-    CC1Args.push_back("-include");
-    CC1Args.push_back("__clang_cuda_runtime_wrapper.h");
-  }
-}
+  if (!CudaInstallation.isValid()) {
+    getDriver().Diag(diag::err_drv_no_cuda_installation);
+    return;
+  }
+
 
 bool Linux::isPIEDefault() const { return getSanitizerArgs().requiresPIE(); }
 
 SanitizerMask Linux::getSupportedSanitizers() const {
   const bool IsX86 = getTriple().getArch() == llvm::Triple::x86;
   const bool IsX86_64 = getTriple().getArch() == llvm::Triple::x86_64;
   const bool IsMIPS64 = getTriple().getArch() == llvm::Triple::mips64 ||
                         getTriple().getArch() == llvm::Triple::mips64el;