[analyzer] Look for allocation site in the parent frames as well as the current one.

Instead of handling edge cases (mostly involving blocks), where we have difficulty finding
an allocation statement, allow the allocation site to be in a parent node.

Previously we assumed that the allocation site can always be found in the same frame
as allocation, but there are scenarios in which an element is leaked in a child
frame but is allocated in the parent.

llvm-svn: 228247