c++: Standardise errors for module_may_redeclare

c++: Standardise errors for module_may_redeclare

Currently different places calling 'module_may_redeclare' all emit very
similar but slightly different error messages, and handle different
kinds of declarations differently.  This patch makes the function
perform its own error messages so that they're all in one place, and
prepares it for use with temploid friends.

gcc/cp/ChangeLog:

	* cp-tree.h (module_may_redeclare): Add default parameter.
	* decl.cc (duplicate_decls): Don't emit errors for failed
	module_may_redeclare.
	(xref_tag): Likewise.
	(start_enum): Likewise.
	* semantics.cc (begin_class_definition): Likewise.
	* module.cc (module_may_redeclare): Clean up logic. Emit error
	messages on failure.

gcc/testsuite/ChangeLog:

	* g++.dg/modules/enum-12.C: Update error message.
	* g++.dg/modules/friend-5_b.C: Likewise.
	* g++.dg/modules/shadow-1_b.C: Likewise.

Signed-off-by: Nathaniel Shead <nathanieloshead@gmail.com>
Reviewed-by: Jason Merrill <jason@redhat.com>