[DWARF] Centralize user_id <-> DWARFDIE conversions

Summary:
The logic for translating a user_id into a DWARFDIE was replicated in
several places. This removes that redundancy and settles on a single
implementation in SymbolFileDWARF.

The reason for choosing that instead of DIERef was that we were
always immediately converting the returned DIERef into a DWARFDIE
anyway, which meant that one had to specify the SymbolFileDWARF argument
twice (once to get the DIERef, and once to get the actual DIE). Also,
passing a higher-level object (SymbolFileDWARF) into a lower-level one
(DIERef) seemed like a less intuitive arrangement than doing things the
other way around.

Reviewers: JDevlieghere, clayborg, aprantl

Subscribers: tberghammer, jankratochvil, lldb-commits

Differential Revision: https://reviews.llvm.org/D61648

llvm-svn: 360246