[DWARF parser] Compress DIEMinimal even further, simplify building DIE tree.

DIE doesn't need to store a pointer to its parent: we can traverse the DIE tree
only with functions getFirstChild() and getSibling(). Parents must be known
only when we construct the tree. Rewrite setDIERelations() procedure in a more
straightforward way, and get rid of lots of now unused DIEMinimal methods.

No functionality change.

llvm-svn: 207563