This patch replaces uses of StringRef::{starts,ends}with with
StringRef::{starts,ends}_with for consistency with
std::{string,string_view}::{starts,ends}_with in C++20.
I'm planning to deprecate and eventually remove
StringRef::{starts,ends}with.
This teaches obj2yaml to dump valid regular (not thin) archives.
This also teaches yaml2obj to recognize archives YAML descriptions,
what allows to craft all different kinds of archives (valid and broken ones).
Differential revision: https://reviews.llvm.org/D89949