[debugger] Resolve global variables.

Searches upward from the current scope for matches. This should find
variables in the global namespace, in relatively-resolved,
explicitly-named namespaces ("ns::Bar"), in explicitly-named
absolutely-resolved names ("::ns::Bar"), and also static members of
classes ("ns::MyClass::kConstant").

This does not yet handle anonymous namespaces.

TEST=unit, manual

Change-Id: Idf402bef76b5abc70d91dc18be18cb0001330e6b
21 files changed