Rewrite and cleanup unique_ptr tests.

This patch almost entirely rewrites the unique_ptr tests. There are a couple
of reasons for this:

A) Most of the *.fail.cpp tests were either incorrect or could be better written
  as a *.pass.cpp test that uses <type_traits> to check if certain operations
  are valid (Ex. Using static_assert(!std::is_copy_constructible_v<T>) instead
  of writing a failure test).

B) [unique.ptr.runtime] has very poor test coverage. Many of the constructors
  and assignment operators have to tests at all. The special members that have
  tests have very few test cases and are typically way out of date.

C) The tests for [unique.ptr.single] and [unique.ptr.runtime] are largely
  duplicates of each other. This means common requirements have two different
  sets of tests in two different test files. This makes the tests harder to
  maintain than if there was a single copy.

To address (A) this patch changes almost all of the *.fail.cpp tests into
.pass.cpp tests using type traits; Allowing the *.fail.cpp tests to be removed.

The address (B) and (C) the tests for [unique.ptr.single] and [unique.ptr.runtime]
have been combined into a single directory, allowing both specializations to share
common tests. Tests specific to the single/runtime specializations are given the
suffix "*.single.pass.cpp" or "*.runtime.pass.cpp".

Finally the unique.ptr test have been moved into the correct directory according
to the standard. Specifically they have been removed from "utilities/memory" into
"utilities/smartptr".

PS. This patch also adds newly written tests for upcoming unique_ptr changes/fixes.
However since these tests don't currently pass they are guarded by the macro
TEST_WORKAROUND_UPCOMING_UNIQUE_PTR_CHANGES. This allows other STL's to validate
the tests before libc++ implements the changes. The relevant libc++ changes should
land in the next week.

llvm-svn: 300388