c++: Don't mix timevar_start and auto_cond_timevar for TV_NAME_LOOKUP [PR116681]

c++: Don't mix timevar_start and auto_cond_timevar for TV_NAME_LOOKUP [PR116681]

We currently ICE upon the following testcase when using -ftime-report

=== cut here ===
template < int> using __conditional_t = int;
template < typename _Iter >
concept random_access_iterator = requires { new _Iter; };
template < typename _Iterator >
struct reverse_iterator {
  using iterator_concept =
    __conditional_t< random_access_iterator< _Iterator>>;
};
void RemoveBottom() {
  int iter;
  for (reverse_iterator< int > iter;;)
      ;
}
=== cut here ===

The problem is that qualified_namespace_lookup does a plain start() of
the TV_NAME_LOOKUP timer (that asserts that the timer is not already
started). However this timer has already been cond_start()'d in the call
stack - by pushdecl - so the assert fails.

This patch simply ensures that we always conditionally start this timer
(which is done in all other places that use it).

	PR c++/116681

gcc/cp/ChangeLog:

	* name-lookup.cc (qualified_namespace_lookup): Use an
	auto_cond_timer instead of using timevar_start and timevar_stop.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/concepts-pr116681.C: New test.