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DWARF: Avoid storing DIERefs in long-lived containers

Authored by labath on Jun 17 2019, 12:32 AM.

Description

DWARF: Avoid storing DIERefs in long-lived containers

Summary:
A user_id_t carries the same information as a DIERef, but it takes up
less space.

Furthermore, DIERef::operator<'s implementation is very
questionable, as it does not take the cu_offset and section fields into
account. Using just the die offset was correct in the days when all
debug info lived in a single section, but since we started supporting
DWO debug info, this was no longer true. The comparison operator could
be fixed, but it seems like using the user_id_t for these purposes is a
better idea overall.

I think this did not cause any bugs, because the only place the
comparison operator was used is in m_function_scope_qualified_name_map,
and this one is local to a dwo file, but I am not 100% sure of that.

Reviewers: clayborg, JDevlieghere

Subscribers: aprantl, lldb-commits

Differential Revision: https://reviews.llvm.org/D63322

llvm-svn: 363528