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.
The clangIndexSerialization target seems to be unused, so it didn't build when I ran `check-all` before landing d1f86c3d. This should resolve the deprecation warning that caused some build bots to fail.