[flang] Merge flang-compiler/f18

This is the initial merge of flang-compiler, which is done in this way
principally to preserve the history and git-blame, without generating a large
number of commits on the first-parent history of LLVM.

If you don't care about the flang history during a bisect remember that you can
supply paths to git-bisect, e.g. `git bisect start clang llvm`.

The history of f18 was rewritten to:

* Put the code under /flang/.
* Linearize the history.
* Rewrite commit messages so that issue and PR numbers point to the old repository.

Credit to Peter Waller for writing the flatten and merge script.

Updates: flang-compiler/f18#876 (submission into llvm-project)
Mailing-list: http://lists.llvm.org/pipermail/llvm-dev/2020-January/137989.html ([llvm-dev] Flang landing in the monorepo - next Monday!)
Mailing-list: http://lists.llvm.org/pipermail/llvm-dev/2019-December/137661.html ([llvm-dev] Flang landing in the monorepo)

Co-authored-by: Peter Waller <peter.waller@arm.com>