[SSAF] Extract the virtual method override relation per TU
A virtual call may dispatch to any override of its callee, so a whole-program
analysis cannot reason about a method's parameters and return value in
isolation. It needs to know which method overrides which, and which slots
that relates. Collect this per TU, so a later pass can join the related
slots into families.
JSON serialization lands separately, so the summary is not writable via
--ssaf-extract-summaries yet.
ยง1 of rdar://179151603
