c++: Fix template class lookup [PR120495, PR115605].

c++: Fix template class lookup [PR120495, PR115605].

In the reported issues, using lookup_template_class () directly on (for example)
the coroutine_handle identifier fails because a class-local  TYPE_DECL is found.
This is because, in the existing code, lookup is called with default parameters
which means that class contexts are examined first.

Fix this, when a context is provided by the caller, by doing lookup in namespace
provided.

	PR c++/120495
	PR c++/115605

gcc/cp/ChangeLog:

	* pt.cc (lookup_template_class): Honour provided namespace contexts
	when looking up class templates.

gcc/testsuite/ChangeLog:

	* g++.dg/coroutines/pr120495.C: New test.
	* g++.dg/pr115605.C: New test.

Signed-off-by: Iain Sandoe <iain@sandoe.co.uk>