c++: optimize push_to_top_level [PR64500]

c++: optimize push_to_top_level [PR64500]

Profiling showed that the loop to save away IDENTIFIER_BINDINGs from open
binding levels was taking 5% of total compilation time in the PR116285
testcase.  This turned out to be because we were unnecessarily trying to do
this for namespaces, whose bindings are found through
DECL_NAMESPACE_BINDINGS, not IDENTIFIER_BINDING.

As a result we would frequently loop through everything in std::, checking
whether it needs to be stored, and never storing anything.

This change actually appears to speed up compilation for the PR116285
testcase by ~20%.

The replaced comments referred either to long-replaced handling of classes
and templates, or to wanting b to point to :: when the loop exits.

	PR c++/64500
	PR c++/116285

gcc/cp/ChangeLog:

	* name-lookup.cc (push_to_top_level): Don't try to store_bindings
	for namespace levels.