[flang] Remove non-alignment based divergences from LLVM formatting.

This only changs the clang-format file and adds documentation
referring to the new process for formatting code. A future commit
will perform a reformatting according to these new formatting
settings.

Original-commit: flang-compiler/f18@02531d788e7550a2455633f87d22e3f30de7a3a7
Reviewed-on: https://github.com/flang-compiler/f18/pull/945