[PATCH] [libcxx] Support UDTs convertible to arithmetic types in <cmath>

[Marshall Clow]

The `__promote` and `lazy_enable_if` changes have already gone into libc++ (I believe) as part of another patch.

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Comment at: include/type_traits:1180
@@ -1182,2 +1179,3 @@
 
-template <class _Tp>
+float __numeric_test(float);
+double __numeric_test(char);
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Is there a reason that these functions are no longer static members?

http://reviews.llvm.org/D5942