functions marked 'nobuiltin'. That approach doesn't play well with LTO, and
there's no harm in marking a call as 'builtin' if it was going to be a builtin
regardless.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@190233 91177308-0d34-0410-b5e6-96231b3b80d8
