swift234

Runtime: add an explicit cast; NFC

This adds an explicit cast of the atomic type to the contained type prior to
comparing it to nullptr.  This is generally unnecessary, however, the Windows
C++ library (msvcprt) from Visual Studio 2014 (WinSDK 10.0.10586.0) has
extensions which makes the conversion ambiguous.  Simply cast the value on all
targets.

In order to reduce the duplication of the type, create a local typedef for the
constant runtime-uniqued metadata pointer type.