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

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@304127 91177308-0d34-0410-b5e6-96231b3b80d8
55 files changed