[ADT] Introduce StringRef::{starts,ends}_width{,_insensitive}

This patch introduces:

  StringRef::starts_with
  StringRef::starts_with_insensitive
  StringRef::ends_with
  StringRef::ends_with_insensitive

to be more compatible with std::string and std::string_view.

I'm planning to deprecate the existing functions in favor of the new
ones.

Differential Revision: https://reviews.llvm.org/D136030