c++: don't ICE on NAMESPACE_DECL inside FUNCTION_DECL

c++: don't ICE on NAMESPACE_DECL inside FUNCTION_DECL

Code like
  void swap() {
    namespace __variant = __detail::__variant;
    ...
  }
create a NAMESPACE_DECL where the CP_DECL_CONTEXT is a FUNCTION_DECL.
DECL_TEMPLATE_INFO fails on NAMESPACE_DECL and therefore must be handled
first in the assertion.

Signed-off-by: Matthias Kretz <m.kretz@gsi.de>
gcc/cp/ChangeLog:

	* module.cc (trees_out::get_merge_kind): NAMESPACE_DECLs also
	cannot have a DECL_TEMPLATE_INFO.

(cherry picked from commit 6bcb6ed5a44b6f271891246ef7ae568bfdc14e9c)