Modernize conversion to bool to the explicit bool conversion operator (library wide).  This fixes http://llvm.org/bugs/show_bug.cgi?id=12058.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@151088 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/include/ios b/include/ios
index 3aa066b..51d5087 100644
--- a/include/ios
+++ b/include/ios
@@ -572,7 +572,8 @@
     typedef typename traits_type::pos_type pos_type;
     typedef typename traits_type::off_type off_type;
 
-    _LIBCPP_ALWAYS_INLINE // explicit
+    _LIBCPP_ALWAYS_INLINE
+        _LIBCPP_EXPLICIT
         operator bool() const {return !fail();}
     _LIBCPP_ALWAYS_INLINE bool operator!() const    {return  fail();}
     _LIBCPP_ALWAYS_INLINE iostate rdstate() const   {return ios_base::rdstate();}