libcxx PATCH - mark integral_constant's methods as noexcept

[Marshall Clow]

This is LWG Issue #2346
	http://cplusplus.github.io/LWG/lwg-active.html#2346

— Marshall


Index: include/type_traits
===================================================================
--- include/type_traits	(revision 201348)
+++ include/type_traits	(working copy)
@@ -236,10 +236,10 @@
     typedef _Tp               value_type;
     typedef integral_constant type;
     _LIBCPP_INLINE_VISIBILITY
-        _LIBCPP_CONSTEXPR operator value_type() const {return value;}
+        _LIBCPP_CONSTEXPR operator value_type() const _NOEXCEPT {return value;}
 #if _LIBCPP_STD_VER > 11
     _LIBCPP_INLINE_VISIBILITY
-         constexpr value_type operator ()() const {return value;}
+         constexpr value_type operator ()() const _NOEXCEPT {return value;}
 #endif
 };