Avoid Clang error about throwing _LIBCPP_ASSERT in noexcept function.

This fixes a couple of tests which produced a warning that a 'throw'
occurred in a noexcept function (by way of _LIBCPP_ASSERT). It does
so by hiding the 'throw' across an opaque function boundary.

This fix isn't ideal, since we still have _LIBCPP_ASSERT's in functions
marked noexcept -- and this problem should be addressed in the future.
However, throwing _LIBCPP_ASSERT is really only meant to allow testing
of the assertions, and is not yet ready for general use.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@328265 91177308-0d34-0410-b5e6-96231b3b80d8

test/libcxx/thread/futures/futures.promise/set_exception.pass.cpp

@@ -11,6 +11,9 @@
 // UNSUPPORTED: libcpp-has-no-threads
 // UNSUPPORTED: c++98, c++03
 
+// MODULES_DEFINES: _LIBCPP_DEBUG_USE_EXCEPTIONS
+// MODULES_DEFINES: _LIBCPP_DEBUG=0
+
 // <future>
 
 // class promise<R>
@@ -18,9 +21,8 @@
 // void set_exception(exception_ptr p);
 // Test that a null exception_ptr is diagnosed.
 
-#define _LIBCPP_ASSERT(x, m) ((x) ? ((void)0) : throw 42)
-
 #define _LIBCPP_DEBUG 0
+#define _LIBCPP_DEBUG_USE_EXCEPTIONS
 #include <future>
 #include <exception>
 #include <cstdlib>
@@ -29,14 +31,14 @@
 
 int main()
 {
+    typedef std::__libcpp_debug_exception ExType;
     {
         typedef int T;
         std::promise<T> p;
         try {
             p.set_exception(std::exception_ptr());
             assert(false);
-        } catch (int const& value) {
-            assert(value == 42);
+        } catch (ExType const&) {
         }
     }
     {
@@ -45,8 +47,7 @@ int main()
         try {
             p.set_exception(std::exception_ptr());
             assert(false);
-        } catch (int const& value) {
-            assert(value == 42);
+        } catch (ExType const&) {
         }
     }
 }