[clang][NFC] Refactor logic for picking standard library on Apple

[ldionne]

Summary

Flip the logic around: always default to libc++ except on older platforms,
instead of defaulting to libstdc++ except on newer platforms. Since roughly
all supported platforms use libc++ now, it makes more sense to make that
the default, and allows the removal of some downstream diff.

[egorzhdan]

LGTM

clang/lib/Driver/ToolChains/Darwin.cpp

if (Ty == types::TY_PP_Asm)
     return types::TY_Asm;
 
   return Ty;
 }
 
 bool MachO::HasNativeLLVMSupport() const { return true; }
 
 ToolChain::CXXStdlibType Darwin::GetDefaultCXXStdlibType() const {
-  // Default to use libc++ on OS X 10.9+ and iOS 7+.
-  if ((isTargetMacOSBased() && !isMacosxVersionLT(10, 9)) ||
-      (isTargetIOSBased() && !isIPhoneOSVersionLT(7, 0)) ||
-      isTargetWatchOSBased())
-    return ToolChain::CST_Libcxx;
-
+  // Use libstdc++ on old targets (OSX < 10.9 and iOS < 7)
+  if ((isTargetMacOSBased() && isMacosxVersionLT(10, 9)) ||
+      (isTargetIOSBased() && isIPhoneOSVersionLT(7, 0)))
     return ToolChain::CST_Libstdcxx;
+
+  // On all other targets, use libc++
+  return ToolChain::CST_Libcxx;
 }
 
 /// Darwin provides an ARC runtime starting in MacOS X 10.7 and iOS 5.0.
 ObjCRuntime Darwin::getDefaultObjCRuntime(bool isNonFragile) const {
   if (isTargetWatchOSBased())
     return ObjCRuntime(ObjCRuntime::WatchOS, TargetVersion);
   if (isTargetIOSBased())
     return ObjCRuntime(ObjCRuntime::iOS, TargetVersion);