Kill deprecated attribute API

Summary:
This kill various depreacated API related to attribute :
 - The deprecated C API attribute based on LLVMAttribute enum.
 - The Raw attribute set format (planned to be removed in 4.0).

Reviewers: bkramer, echristo, mehdi_amini, void

Subscribers: mehdi_amini, llvm-commits

Differential Revision: https://reviews.llvm.org/D23039

llvm-svn: 286062