Fix iOSDarwin()'s doc comment.

[varungandhi-apple]

Summary

I saw the doc comment using the Option + Click in Xcode and I was
confused by the fact that tvOS wasn't listed. The method definition
also doesn't make it clear that the check for tvOS is taking place as
the tvOS check is hidden in the isiOS() check.

[rjmccall]

Might as well change "OS X" to "macOS" while you're at it.

[varungandhi-apple]

Addressed John's suggestion to replace OS X -> macOS.

[rjmccall]

LGTM

llvm/include/llvm/ADT/Triple.h

public:
   bool isWatchOS() const {
     return getOS() == Triple::WatchOS;
   }
 
   bool isWatchABI() const {
     return getSubArch() == Triple::ARMSubArch_v7k;
   }
 
-  /// isOSDarwin - Is this a "Darwin" OS (OS X, iOS, or watchOS).
+  /// isOSDarwin - Is this a "Darwin" OS (macOS, iOS, tvOS or watchOS).
   bool isOSDarwin() const {
     return isMacOSX() || isiOS() || isWatchOS();
   }
 
   bool isSimulatorEnvironment() const {
     return getEnvironment() == Triple::Simulator;
   }