[docs][Remarks] Add documentation for remarks in LLVM

This adds documentation that describes remarks in LLVM.

It aims at explaining what remarks are, how to enable them, and what
users can do with the different modes.

It lists all the available flags in LLVM (excluding clang), and
describes the expected YAML structure as well as the tools that support
the YAML format today.

Differential Revision: https://reviews.llvm.org/D64355

llvm-svn: 365578