Add an option group for deprecated warnings. Add the removed

-fslp-vectorize-aggressive and -fno-slp-vectorize-aggressive flags back
under this group and test for the warning. Document the future removal
in the ReleaseNotes.

Differential Revision: https://reviews.llvm.org/D34926


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@306965 91177308-0d34-0410-b5e6-96231b3b80d8