StartFunction enters a release cleanup for ns_consumed arguments in
ARC, so we need to balance that somehow.  We could teach StartFunction
that it's emitting a delegating function, so that the cleanup is
unnecessary, but that would be invasive and somewhat fraught.  We could
balance the consumed argument with an extra retain, but clearing the
original variable should be easier to optimize and avoid some extra work
at -O0.  And there shouldn't be any difference as long as nothing else
uses the argument, which should always be true for the places we emit
delegate arguments.

Fixes PR 27887.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@287291 91177308-0d34-0410-b5e6-96231b3b80d8