[OptRemarks] Make OptRemarks more generic: rename OptRemarks to Remarks

Getting rid of the name "optimization remarks" for anything that
involves handling remarks on the client side.

It's safer to do this now, before we get stuck with that name in all the
APIs and public interfaces we decide to export to users in the future.

This renames llvm/tools/opt-remarks to llvm/tools/remarks-shlib, and now
generates `libRemarks.dylib` instead of `libOptRemarks.dylib`.

Differential Revision: https://reviews.llvm.org/D58535

llvm-svn: 355439