fix typos in comments


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@216424 91177308-0d34-0410-b5e6-96231b3b80d8
musttail: Don't eliminate varargs packs if there is a forwarding call

Also clean up and beef up this grep test for the feature.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@216425 91177308-0d34-0410-b5e6-96231b3b80d8