[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
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@344162
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