[flang] Flang relicense: LICENSE file and one change (flang-compiler/f18#867)

Add new license text to runtime/magic-numbers.h, which is unusual
in that it is the only .h file in flang that is intended to be
included in either C or Fortran files. This requires special
handling of the license text.

Original-commit: flang-compiler/f18@61bb9db531ad4fbe8fda9978a6b7754cb86323c2
Reviewed-on: https://github.com/flang-compiler/f18/pull/867