[analyzer] Mark symbol values as dead in the environment.

[analyzer] Mark symbol values as dead in the environment.

This allows us to properly remove dead bindings at the end of the top-level
stack frame, using the ReturnStmt, if there is one, to keep the return value
live. This in turn removes the need for a check::EndPath callback in leak
checkers.

This does cause some changes in the path notes for leak checkers. Previously,
a leak would be reported at the location of the closing brace in a function.
Now, it gets reported at the last statement. This matches the way leaks are
currently reported for inlined functions, but is less than ideal for both.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@168066 91177308-0d34-0410-b5e6-96231b3b80d8