c++/modules: Assume header bindings are global module

While stepping through some code I noticed that we do some extra work
(finding the originating module decl, stripping the template, and
inspecting the attached-ness) for every declaration taken from a header
unit.  This doesn't seem necessary though since no declaration in a
header unit can be attached to anything but the global module, so we can
just assume that global_p will be true.

This was the original behaviour before I removed this assumption while
refactoring for r15-2807-gc592310d5275e0.

gcc/cp/ChangeLog:

	* module.cc (module_state::read_cluster): Assume header module
	declarations will require GM merging.

Signed-off-by: Nathaniel Shead <nathanieloshead@gmail.com>