unique_ptrify PathDiagnostic::setEndOfPath's argument

Again, if shared ownership is the right model here (I assume it is,
given graph algorithms & such) this could be tidied up (the 'release'
call removed in favor of something safer) by having
IntrunsiveRefCntPointer constructible from a unique_ptr.

(& honestly I'd probably favor taking a page out of shared_ptr's book,
allowing implicit construction from a unique_ptr rvalue, and only allow
explicit from a raw pointer - currently IntrusiveRefCntPointer can
implicitly own from a raw pointer, which seems unsafe)