Shorten DiagnosticInfoOptimizationRemark* to OptimizationRemark*. NFC

With the new streaming interface, these class names need to be typed a
lot and it's way too looong.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@282544 91177308-0d34-0410-b5e6-96231b3b80d8
7 files changed