c++: Allow exporting a typedef redeclaration [PR102341]

c++: Allow exporting a typedef redeclaration [PR102341]

A typedef doesn't create a new entity, and thus should be allowed to be
exported even if it has been previously declared un-exported. See the
example in [module.interface] p6:

  export module M;
  struct S { int n; };
  typedef S S;
  export typedef S S;             // OK, does not redeclare an entity

	PR c++/102341

gcc/cp/ChangeLog:

	* decl.cc (duplicate_decls): Allow exporting a redeclaration of
	a typedef.

gcc/testsuite/ChangeLog:

	* g++.dg/modules/export-1.C: Adjust test.
	* g++.dg/modules/export-2_a.C: New test.
	* g++.dg/modules/export-2_b.C: New test.

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