[llvm] Migrate {starts,ends}with_insensitive to {starts,ends}_with_insensitive (NFC)

This patch migrates uses of StringRef::{starts,ends}with_insensitive
to StringRef::{starts,ends}_with_insensitive so that we can use names
similar to those used in std::string_view.  I'm planning to deprecate
StringRef::{starts,ends}with_insensitive once the migration is
complete across the code base.

Differential Revision: https://reviews.llvm.org/D150426