[clang-format] add case aware include sorting

* Adds an option to [clang-format] which sorts
  headers in an alphabetical manner using case
  only for tie-breakers. The options is off by
  default in favor of the current ASCIIbetical
  sorting style.

Reviewed By: curdeius, HazardyKnusperkeks

Differential Revision: https://reviews.llvm.org/D95017