Reland: [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
It exposes a C API which takes a handler that is called with the remark
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
Differential Revision: https://reviews.llvm.org/D52776
Fixed the tests by removing the usage of C++11 strings, which seems not
to be supported by gcc 4.8.4 if they're used as a macro argument.