Overhaul unique_ptr - Implement LWG 2801, 2905, 2520.

This patch overhauls both specializations of unique_ptr while implementing
the following LWG issues:

* LWG 2801 - This issue constrains unique_ptr's constructors when the deleter type
  is not default constructible. Additionally it adds SFINAE conditions
  to unique_ptr<T[]>::unique_ptr(Up).

* LWG 2905 - This issue reworks the unique_ptr(pointer, /* see below */ deleter)
  constructors so that they correctly SFINAE when the deleter argument cannot
  be used to construct the stored deleter.

* LWG 2520 - This issue fixes initializing unique_ptr<T[]> from nullptr.
  Libc++ had previously implemented this issue, but the suggested resolution
  still broke initialization from NULL. This patch re-works the
  unique_ptr<T[]>(Up, deleter) overloads so that they accept NULL as well
  as nullptr.

llvm-svn: 300406