[PATCH] [libcxx] Use __decltype instead of __typeof__

[Marshall Clow]

================
Comment at: include/__config:29
@@ -24,1 +28,3 @@
+#endif
+
 #if !_WIN32
----------------
I'm not really pleased with the idea of switching off the clang version. This kind of stuff grows over time.

If there is a better test, I'd rather use that. 
If not, well...

http://reviews.llvm.org/D10426

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