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/istream b/include/istream
index c220532..7312425 100644
--- a/include/istream
+++ b/include/istream
@@ -256,7 +256,7 @@
 //    ~sentry() = default;
 
     _LIBCPP_INLINE_VISIBILITY
-    // explicit
+        _LIBCPP_EXPLICIT
         operator bool() const {return __ok_;}
 };