[D] Move building of typeof(null) values to a common function.

[D] Move building of typeof(null) values to a common function.

gcc/d/ChangeLog:

	* d-codegen.cc (build_typeof_null_value): New function.
	* d-tree.h (build_typeof_null_value): Declare.
	* d-convert.cc (convert_expr): Use build_typeof_null_value.
	* expr.cc (ExprVisitor::visit(NullExp)): Likewise.

From-SVN: r267955