[flang] Clean up Makefile, add reformat target.

Now the only targets in Makefile are Debug, Release, and reformat.
"make reformat" reformats all C++ source in place.

Also includes the result of "make reformat". Most of the changes were
just moving the "main" include up.

Original-commit: flang-compiler/f18@cf7d3a7afb650b5fc952894136dd18e5551ac578