Implement uncaught_exceptions() using the newly added hooks in libc++abi, when available

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@238846 91177308-0d34-0410-b5e6-96231b3b80d8

test/std/language.support/support.exception/uncaught/uncaught_exceptions.pass.cpp

@@ -0,0 +1,45 @@
+//===----------------------------------------------------------------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is dual licensed under the MIT and the University of Illinois Open
+// Source Licenses. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+// test uncaught_exceptions
+
+#include <exception>
+#include <cassert>
+
+struct A
+{
+    ~A()
+    {
+        assert(std::uncaught_exceptions() > 0);
+    }
+};
+
+struct B
+{
+    B()
+    {
+        // http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_defects.html#475
+        assert(std::uncaught_exceptions() == 0);
+    }
+};
+
+int main()
+{
+    try
+    {
+        A a;
+        assert(std::uncaught_exceptions() == 0);
+        throw B();
+    }
+    catch (...)
+    {
+        assert(std::uncaught_exception() == 0);
+    }
+    assert(std::uncaught_exceptions() == 0);
+}