[libc++] Implement exception_ptr on Windows

Summary:
This patch implements exception_ptr on Windows using the `__ExceptionPtrFoo` functions provided by MSVC.

The `__ExceptionPtrFoo` functions are defined inside the C++ standard library, `msvcprt`, which is unfortunate because it requires libc++ to link to the MSVC STL. However this doesn't seem to cause any immediate problems. However to be safe I kept all usages within the libc++ dylib so that user programs wouldn't have to link to MSVCPRT as well.

Note there are still 2 outstanding exception_ptr/nested_exception test failures.

* `current_exception.pass.cpp` needs to be rewritten for the Windows exception_ptr semantics which copy the exception every time.
* `rethrow_if_nested.pass.cpp` need investigation. It hits a stack overflow, likely from recursion.

This patch also gets most of the `<future>` tests passing as well.

Reviewers: mclow.lists, compnerd, bcraig, rmaprath, majnemer, BillyONeal, STL_MSFT

Subscribers: mgorny, cfe-commits

Differential Revision: https://reviews.llvm.org/D32927

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@302393 91177308-0d34-0410-b5e6-96231b3b80d8

test/std/language.support/support.exception/propagation/rethrow_exception.pass.cpp

@@ -7,9 +7,6 @@
 //
 //===----------------------------------------------------------------------===//
 
-// exception_ptr has not been implemented on Windows
-// XFAIL: LIBCXX-WINDOWS-FIXME
-
 // UNSUPPORTED: libcpp-no-exceptions
 // <exception>
 
@@ -49,7 +46,12 @@ int main()
         }
         catch (const A& a)
         {
+#ifndef _LIBCPP_ABI_MICROSOFT
             assert(A::constructed == 1);
+#else
+            // On Windows the exception_ptr copies the exception
+            assert(A::constructed == 2);
+#endif
             assert(p != nullptr);
             p = nullptr;
             assert(p == nullptr);
@@ -58,4 +60,5 @@ int main()
         }
         assert(A::constructed == 0);
     }
+    assert(A::constructed == 0);
 }