gdb: add and use signatured_type_up

gdb: add and use signatured_type_up

Add an alias for std::unique_ptr<signatured_type> and use it where
possible.

gdb/ChangeLog:

	* dwarf2/read.h (signatured_type_up): New, use where possible.

Change-Id: I5a41e8345551434c8beeb9f269b03bdcf27989be