[ADT] Add relation operators for Optional

Summary: Make Optional's behavior the same as the coming std::optional.

Reviewers: dblaikie

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D23178

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@278397 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed