IRGen: Add optnone attribute on function during O0

Amongst other, this will help LTO to correctly handle/honor files
compiled with O0, helping debugging failures.
It also seems in line with how we handle other options, like how
-fnoinline adds the appropriate attribute as well.

Differential Revision: https://reviews.llvm.org/D28404

llvm-svn: 304127
diff --git a/clang/lib/Frontend/CompilerInvocation.cpp b/clang/lib/Frontend/CompilerInvocation.cpp
index 859e8ba..7d7e7d4 100644
--- a/clang/lib/Frontend/CompilerInvocation.cpp
+++ b/clang/lib/Frontend/CompilerInvocation.cpp
@@ -534,6 +534,7 @@
 
   Opts.DisableLLVMPasses = Args.hasArg(OPT_disable_llvm_passes);
   Opts.DisableLifetimeMarkers = Args.hasArg(OPT_disable_lifetimemarkers);
+  Opts.DisableO0ImplyOptNone = Args.hasArg(OPT_disable_O0_optnone);
   Opts.DisableRedZone = Args.hasArg(OPT_disable_red_zone);
   Opts.ForbidGuardVariables = Args.hasArg(OPT_fforbid_guard_variables);
   Opts.UseRegisterSizedBitfieldAccess = Args.hasArg(