[lldb/DWARF] Simplify DWARFDebugInfoEntry::LookupAddress

Summary:
This method was doing a lot more than it's only caller needed
(DWARFDIE::LookupDeepestBlock) needed, so I inline it into the caller,
and remove any code which is not actually used. This includes code for
searching for the deepest function, and the code for working around
incomplete DW_AT_low_pc/high_pc attributes on a compile unit DIE (modern
compiler get this right, and this method is called on function DIEs
anyway).

This also improves our llvm consistency, as llvm::DWARFDebugInfoEntry is
just a very simple struct with no nontrivial logic.

Reviewers: JDevlieghere, aprantl

Subscribers: lldb-commits

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D72920