RAPIDJSON_NOEXCEPT_ASSERT() should never throw.

clang warns about throwing from RAPIDJSON_NOEXCEPT_ASSERT() in a nothrow
context.

If RAPIDJSON_ASSERT() throws it can never be used for _NOEXCEPT_ASSERT(),
so use C assert() instead.

Finally (and originally), since RAPIDJSON_ASSERT() in "unittest.h" throws,
make it define RAPIDJSON_ASSERT_THROWS for RAPIDJSON_NOEXCEPT_ASSERT() to
now do the right thing.

test/unittest/unittest.h

@@ -122,6 +122,9 @@ public:
 
 #ifndef RAPIDJSON_ASSERT
 #define RAPIDJSON_ASSERT(x) (!(x) ? throw AssertException(RAPIDJSON_STRINGIFY(x)) : (void)0u)
+#ifndef RAPIDJSON_ASSERT_THROWS
+#define RAPIDJSON_ASSERT_THROWS
+#endif
 #endif
 
 class Random {