[libcxx] func.wrap.func.con: Unset function before destroying anything

Be defensive against a reentrant std::function::operator=(nullptr_t), in case
the held function object has a non-trivial destructor.  Destroying the function
object in-place can lead to the destructor being called twice.

Patch by Duncan P. N. Exon Smith. C++03 support by Volodymyr Sapsai.

rdar://problem/32836603

Reviewers: EricWF, mclow.lists

Reviewed By: mclow.lists

Subscribers: cfe-commits, arphaman

Differential Revision: https://reviews.llvm.org/D34331

llvm-svn: 330885