Micro-optimization to avoid creating temporary path

Now that path::operator/=(basic_string_view<value_type>) works directly
from the string argument, instead of constructing a temporary path from
the string, it's potentially more efficient to do 'path(x) /= s' instead
of 'x / s'. This changes the only relevant place in the library.

	* src/filesystem/std-dir.cc (filesystem::_Dir::advance): Append
	string to lvalue to avoid creating temporary path.

From-SVN: r267236

libstdc++-v3/ChangeLog

 2018-12-18  Jonathan Wakely  <jwakely@redhat.com>
 
+	* src/filesystem/std-dir.cc (filesystem::_Dir::advance): Append
+	string to lvalue to avoid creating temporary path.
+
 	* include/bits/fs_path.h (path::compare(const string_type&))
 	(path::compare(const value_type*)): Add noexcept and construct a
 	string view to compare to instead of a path.

libstdc++-v3/src/filesystem/std-dir.cc

@@ -61,7 +61,9 @@ struct fs::_Dir : _Dir_base
   {
     if (const auto entp = _Dir_base::advance(skip_permission_denied, ec))
       {
-	entry = fs::directory_entry{path / entp->d_name, get_file_type(*entp)};
+	auto name = path;
+	name /= entp->d_name;
+	entry = fs::directory_entry{name, get_file_type(*entp)};
 	return true;
       }
     else if (!ec)