[Threading][Windows] Undefine Yield and ERROR.

`windows.h` defines a couple of very unhelpfully named macros.
Undefine them after including it.

rdar://90776105

Author: al45tair

include/swift/Threading/Impl/Win32.h

@@ -21,6 +21,10 @@
 #define NOMINMAX
 #include <windows.h>
 
+// <windows.h> defines some unhelpful macros
+#undef Yield
+#undef ERROR
+
 #include <atomic>
 
 namespace swift {