[analyzer] [NFC] Remove most usages of getEndPath

getEndPath is a problematic API, because it's not clear when it's called
(hint: not always at the end of the path), it crashes at runtime with
more than one non-nullptr returning implementation, and diagnostics
internal depend on it being called at some exact place.

However, most visitors don't actually need that: all they want is a
function consistently called after all nodes are traversed, to perform
finalization and to decide whether invalidation is needed.

Differential Revision: https://reviews.llvm.org/D48042

llvm-svn: 334540