[Remarks] Add a new Remark / RemarkParser abstraction

This adds a Remark class that allows us to share code when working with
remarks.

The C API has been updated to reflect this. Instead of the parser
generating C structs, it's now using a C++ object that is used through
opaque pointers in C. This gives us much more flexibility on what
changes we can make to the internal state of the object and interacts
much better with scenarios where the library is used through dlopen.

* C API updates:
  * move from C structs to opaque pointers and functions
  * the remark type is now an enum instead of a string
* unit tests updates:
  * use mostly the C++ API
  * keep one test for the C API
  * rename to YAMLRemarksParsingTest
* a typo was fixed: AnalysisFPCompute -> AnalysisFPCommute.
* a new error message was added: "expected a remark tag."
* llvm-opt-report has been updated to use the C++ parser instead of the
C API

Differential Revision: https://reviews.llvm.org/D59049

llvm-svn: 356491