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/exception b/include/exception
index dcef3e5..f7c3b70 100644
--- a/include/exception
+++ b/include/exception
@@ -132,7 +132,7 @@
     ~exception_ptr() _NOEXCEPT;
 
     _LIBCPP_INLINE_VISIBILITY
-    // explicit
+    _LIBCPP_EXPLICIT
         operator bool() const _NOEXCEPT {return __ptr_ != nullptr;}
 
     friend _LIBCPP_INLINE_VISIBILITY