Support extracting the current exception count on MSVC2015.

The MSVC runtime was heavily refactored, so the internal function we call to
get the current exception count changed for newer MSVC versions (as did the
offset).

c++/src/kj/exception.c++

@@ -550,23 +550,31 @@ uint uncaughtExceptionCount() {
 
 #elif _MSC_VER
 
-#if 0
-// TODO(msvc): The below was copied from:
-//     https://github.com/panaseleus/stack_unwinding/blob/master/boost/exception/uncaught_exception_count.hpp
-//   Alas, it doesn not appera to work on MSVC2015. The linker claims _getptd() doesn't exist.
+#if _MSC_VER >= 1900
+// MSVC14 has a refactored CRT, so the appropriate runtime function and offsets changed.
+// See https://svn.boost.org/trac/boost/ticket/10158 for a brief discussion.
+extern "C" char *__cdecl __vcrt_getptd();
+
+uint uncaughtExceptionCount() {
+  return *reinterpret_cast<uint*>(__vcrt_getptd() + (sizeof(void*) == 8 ? 0x38 : 0x1c));
+}
+
+#elif _MSC_VER >= 1400
+// The below was copied from:
+// https://github.com/panaseleus/stack_unwinding/blob/master/boost/exception/uncaught_exception_count.hpp
 
 extern "C" char *__cdecl _getptd();
 
 uint uncaughtExceptionCount() {
   return *reinterpret_cast<uint*>(_getptd() + (sizeof(void*) == 8 ? 0x100 : 0x90));
 }
-#endif
-
+#else
 uint uncaughtExceptionCount() {
   // Since the above doesn't work, fall back to uncaught_exception(). This will produce incorrect
   // results in very obscure cases that Cap'n Proto doesn't really rely on anyway.
   return std::uncaught_exception();
 }
+#endif
 
 #else
 #error "This needs to be ported to your compiler / C++ ABI."