Add DWARFUnit::getNumDIEs() and getDIEIndex()

Parsed DIEs are stored in a vector and that makes it easy to get their
indices. Having easy access to a DIE's index makes it possible to use
arrays or vectors to efficiently store/access DIE related information.

There's no test for that new functionality (I don't see how to test
it standalone), but it'll be used in a subsequent dsymutil commit.

llvm-svn: 227381