[analyzer] When we look for the last stmt in a function, skip implicit dtors.

[analyzer] When we look for the last stmt in a function, skip implicit dtors.

When exiting a function, the analyzer looks for the last statement in the
function to see if it's a return statement (and thus bind the return value).
However, the search for "the last statement" was accepting statements that
were in implicitly-generated inlined functions (i.e. destructors). So we'd
go and get the statement from the destructor, and then say "oh look, this
function had no explicit return...guess there's no return value". And /that/
led to the value being returned being declared dead, and all our leak
checkers complaining.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@162791 91177308-0d34-0410-b5e6-96231b3b80d8