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.

Note that the llvm/ directory has migrated in commit
6c3ea866.

I'll post a separate patch to deprecate
StringRef::{starts,ends}with_insensitive.

Differential Revision: https://reviews.llvm.org/D150506