SIL: support non-clang compilers again

The toolchain has traditionally been buildable with clang, cl (MSVC),
and gcc.  `__has_feature` is a clang-specific directive, ensure that we
are portable to other compilers by providing a definition of the macro
incase it is not available (which also includes older clang releases).

Author: compnerd

include/swift/SIL/BridgedSwiftObject.h

@@ -20,6 +20,10 @@
 
 #include <stdint.h>
 
+#if !defined(__has_feature)
+#define __has_feature(feature) 0
+#endif
+
 // TODO: These macro definitions are duplicated in Visibility.h. Move
 // them to a single file if we find a location that both Visibility.h and
 // BridgedSwiftObject.h can import.