[flang] Improve include file sorting.

[flang] Improve include file sorting.

Includes like "../dir/file.h" should sort after local includes.
This change fixes that and applies the new formatting.

Now the order (in reverse) is:
- system includes
- includes from llvm or clang (this is from the default IncludeCategories)
- includes of ../something
- everything else

Original-commit: flang-compiler/f18@324643d63a1842ddfea9ea2a70ebb363f9ced8cc
Reviewed-on: https://github.com/flang-compiler/f18/pull/52