[flang] Initial work on the representation of types.

Still work to do for derived types components, array specs, type-bound
procedures, etc.

Added executable type-test to the cmake file which exercises some of the
basic functionality. Modified the Makefile so that "make Debug" does a
cmake build.

Original-commit: flang-compiler/f18@02e8c4c867685959f8623ca8a4c2277756ecbe6c
Reviewed-on: https://github.com/flang-compiler/f18/pull/3
Tree-same-pre-rewrite: false