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/__mutex_base b/include/__mutex_base
index 5568765..5410272 100644
--- a/include/__mutex_base
+++ b/include/__mutex_base
@@ -207,7 +207,7 @@
     _LIBCPP_INLINE_VISIBILITY
     bool owns_lock() const {return __owns_;}
     _LIBCPP_INLINE_VISIBILITY
-//    explicit
+    _LIBCPP_EXPLICIT
         operator bool () const {return __owns_;}
     _LIBCPP_INLINE_VISIBILITY
     mutex_type* mutex() const {return __m_;}