gccrs: [E0124] field x is already declared in struct

gccrs: [E0124] field x is already declared in struct

Refactored error message for more
than one duplicate fields.

gcc/rust/ChangeLog:

	* hir/rust-ast-lower-base.cc (struct_field_name_exists):
	called error function.

gcc/testsuite/ChangeLog:

	* rust/compile/bad_pub_enumitems.rs: changed comment to pass test cases.
	* rust/compile/dup_fields.rs: likewise.
	* rust/execute/same_field_name.rs: New test.

Signed-off-by: Muhammad Mahad <mahadtxt@gmail.com>