[flang] Add MakeTypes to build types using visitor.

This replaces the manual walking code in test-type.cc. It walks the
parse tree and constructs DeclTypeSpec and DerivedTypeDef objects for
each type encountered. It's in a new file, make-types.cc, so that
failing compilations complete faster.

Handle more parts of derived type definitions.

Also some minor cleanups in type.h and type.cc.

Original-commit: flang-compiler/f18@859e7e2665e0d66c583ffcb7c74265813c25967c
Reviewed-on: https://github.com/flang-compiler/f18/pull/23
Tree-same-pre-rewrite: false