While using dwarf5 `.debug_names` in internal large targets, we noticed
a performance issue (around 10 seconds delay) while `lldb-vscode` tries
to show `scopes` for a compile unit. Profiling shows the bottleneck is
inside `DebugNamesDWARFIndex::GetGlobalVariables` which linearly search
all index entries belongs to a compile unit.
This patch improves the performance by using the compile units list to
filter first before checking index entries. This significantly improves
the performance (drops from 10 seconds => under 1 second) in the split
dwarf situation because each compile unit has its own named index.
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Co-authored-by: jeffreytan81 <jeffreytan@fb.com>
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