[flang] Added CMakeLists changes, moved config and made test scripts compatible.

All Fortran tests are now run in lit, except Preprocessing tests flang-compiler/f18#1052
Preprocessing tests are a separate kind of test, so will be sorted out
later.

Original-commit: flang-compiler/f18@3f99d35f3d4f57f704fd91a0d6ba8af308e900b3
Reviewed-on: https://github.com/flang-compiler/f18/pull/1027