[dwarfdump] Resolve also variable specifications/abstract_origins.

DW_AT_specification and DW_AT_abstract_origin resolving was only performed
on subroutine DIEs because it used the getSubroutineName method. Introduce
a more generic getName() and use it to dump the reference attributes.

Testcases have been updated to check the printed names instead of the offsets
except when the name could be ambiguous.

Reviewers: dblaikie, samsonov

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D5625

llvm-svn: 219506