Debug Info: create scope children DIEs when the scope DIE is not null.

We try to create the scope children DIEs after we create the scope DIE. But
to avoid emitting empty lexical block DIE, we first check whether a scope
DIE is going to be null, then create the scope children if it is not null.
From the number of children, we decide whether to actually create the scope DIE.

This patch also removes an early exit which checks for a special condition.
It also removes deletion of un-used children DIEs that are generated
because we used to generate children DIEs before the scope DIE.

Deletion of un-used children DIEs may cause problem because we sometimes keep
created DIEs in a member variable of a CU.