[Target] Make a copy of TargetOptions feature list before sorting during CodeGen

Currently CodeGen is calling std::sort on the features vector in TargetOptions for every function, but I don't think CodeGen should be modifying TargetOptions.

Differential Revision: https://reviews.llvm.org/D40228

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@319195 91177308-0d34-0410-b5e6-96231b3b80d8