findRecursively follows DW_AT_specification and DW_AT_abstract_origin
references, but not DW_AT_signature. As far as I can tell, there is no
fundamental difference between these attributes that would make this
behavior desirable, and this just seems like a consequence of the fact
that this attribute is newer. This patch aims to change that.

The motivation is some code in lldb, which assumes that it can construct
a qualified name of a type by just walking the parent chain and looking
at the name attribute. This works for "regular" debug info, even when
some of the DIEs are just forward declarations, but it breaks in the
presence of type units, because of the need to explicitly resolve the
signature reference.

While LLDB does not use the llvm's DWARFDie class (yet?), this seems
like a very important change in the overall API, and any divergance here
would complicate eventual reunification, which is why I am making the
change in the llvm API first. Nonetheless, I think this change is
beneficial in llvm as well, as it allows us to remove the explicit
DW_AT_signature resolution in the DWARFTypePrinter.