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.
git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@300406 91177308-0d34-0410-b5e6-96231b3b80d8
a4fd0c9d61