Add nodiscard to std::vector<bool>::empty()

We already added it to the std::vector primary template.

	* include/bits/stl_bvector.h (vector<bool>::empty()): Add nodiscard
	attribute.

From-SVN: r270648

libstdc++-v3/ChangeLog

@@ -1,5 +1,8 @@
 2019-04-29  Jonathan Wakely  <jwakely@redhat.com>
 
+	* include/bits/stl_bvector.h (vector<bool>::empty()): Add nodiscard
+	attribute.
+
 	* include/bits/stl_iterator_base_types.h (_Iter_base): Remove unused
 	class template and partial specialization.
 

libstdc++-v3/include/bits/stl_bvector.h

@@ -878,7 +878,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CONTAINER
       { return size_type(const_iterator(this->_M_impl._M_end_addr(), 0)
 			 - begin()); }
 
-      bool
+      _GLIBCXX_NODISCARD bool
       empty() const _GLIBCXX_NOEXCEPT
       { return begin() == end(); }