Update ReleaseNotes to remove empty sections.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/branches/release_35@217216 91177308-0d34-0410-b5e6-96231b3b80d8
Merge remote-tracking branch 'release_35/release_35'
Merge remote-tracking branch 'origin/master' into clang-omp