[flang] Replace raw pointers with std::unique_ptr.

This simplifies the "delete and set to null" pattern to just a call to
reset(), and "assign and set to null" to a call to std::move.

The ideal would be a custom deleter that throws an error when an
implicit delete happens but not when reset() is called, but I'm not sure
that's possible.

Original-commit: flang-compiler/f18@f618fc729a5b92b036437ec9828788823ce88f13
Reviewed-on: https://github.com/flang-compiler/f18/pull/23