Update Release Notes for 3.4.1

git-svn-id: https://llvm.org/svn/llvm-project/cfe/branches/release_34@208599 91177308-0d34-0410-b5e6-96231b3b80d8
Merge branch 'release_34' into eero_release_34
Merge branch 'eero_release_34' into pipe-period_operator