During jump-scope checking, build an ExprWithCleanups immediately

During jump-scope checking, build an ExprWithCleanups immediately
into the enclosing scope;  this is a more accurate model but is
(I believe) unnecessary in my test case due to other flaws.
However, one of those flaws is now intentional:  blocks which
appear in return statements can be trivially observed to not
extend in lifetime past the return, and so we can allow a jump
past them.  Do the necessary magic in IR-generation to make
this work.

llvm-svn: 164589