We represent AST nodes in ZIR as relative to a "baseline" node so that
source locations within unchanged declarations/types do not get
invalidated by incremental updates. However, we were missing a bit of
granualarity in container types: because container types have
declarations, but type resolution is only re-run if the fields change,
there is no single baseline node in a type declaration relative to which
the fields' AST nodes do not change. Instead, we need to track a few
different baseline nodes, and put the right one in the `LazySrcLoc`.
Matthew Lugg
ac77c23aff