Optimize filesystem::path::parent_path()

Parsing a complete string is more efficient than appending each
component one-by-one.

	* src/c++17/fs_path.cc (path::parent_path()): Create whole path at
	once instead of building it iteratively.

From-SVN: r271754

libstdc++-v3/ChangeLog

 2019-05-29  Jonathan Wakely  <jwakely@redhat.com>
 
+	* src/c++17/fs_path.cc (path::parent_path()): Create whole path at
+	once instead of building it iteratively.
+
 	* testsuite/util/testsuite_api.h: Remove names of unused parameters.
 
 	PR libstdc++/85494 use rdseed and rand_s in std::random_device

libstdc++-v3/src/c++17/fs_path.cc

@@ -1523,11 +1523,9 @@ path::parent_path() const
     __ret = *this;
   else if (_M_cmpts.size() >= 2)
     {
-      for (auto __it = _M_cmpts.begin(), __end = std::prev(_M_cmpts.end());
-	   __it != __end; ++__it)
-	{
-	  __ret /= *__it;
-	}
+      const auto parent = std::prev(_M_cmpts.end(), 2);
+      const auto len = parent->_M_pos + parent->_M_pathname.length();
+      __ret.assign(_M_pathname.substr(0, len));
     }
   return __ret;
 }