[OptRemarks] Add library for parsing optimization remarks

Add a library that parses optimization remarks (currently YAML, so based
on the YAMLParser).

The goal is to be able to provide tools a remark parser that is not
completely dependent on YAML, in case we decide to change the format
later.

It exposes a C API which takes a handler that is called with the remark
structure.

It adds a libLLVMOptRemark.a static library, and it's used in-tree by
the llvm-opt-report tool (from which the parser has been mostly moved
out).

Differential Revision: https://reviews.llvm.org/D52776

llvm-svn: 344162