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

llvm-svn: 151088
Cr-Mirrored-From: sso://chromium.googlesource.com/_direct/external/github.com/llvm/llvm-project
Cr-Mirrored-Commit: f2f2d8b98d7a7ae89f4c646be6dea7730b6409e0
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