Records protocol requirement associated with a particular type in
the constraint system, also useful to track parent conformance
for conditional requirements.
It was used for unresolved member and `.dynamicType` references
as well as a couple of other places, but now unresolved member
references no longer need that due to new implicit "chain result"
AST node and other places could use more precise locators
e.g. new `.dynamicType` locator or `sequence element` for `for in`
loops.
