gdb: introduce displaced_step_closure_up type alias

gdb: introduce displaced_step_closure_up type alias

To help with readability, add the type displaced_step_closure_up, an
alias for std::unique_ptr<displaced_step_closure>, and use it throughout
the code base.

gdb/ChangeLog:

	* aarch64-tdep.c (aarch64_displaced_step_copy_insn): Use
	displaced_step_closure_up.
	* aarch64-tdep.h (aarch64_displaced_step_copy_insn): Likewise.
	(struct displaced_step_closure_up):
	* amd64-tdep.c (amd64_displaced_step_copy_insn): Likewise.
	* amd64-tdep.h (amd64_displaced_step_copy_insn): Likewise.
	* arm-linux-tdep.c (arm_linux_displaced_step_copy_insn):
	Likewise.
	* gdbarch.sh (displaced_step_copy_insn): Likewise.
	* gdbarch.c, gdbarch.h: Re-generate.
	* i386-linux-tdep.c (i386_linux_displaced_step_copy_insn): Use
	displaced_step_closure_up.
	* i386-tdep.c (i386_displaced_step_copy_insn): Likewise.
	* i386-tdep.h (i386_displaced_step_copy_insn): Likewise.
	* infrun.h (displaced_step_closure_up): New type alias.
	(struct displaced_step_inferior_state) <step_closure>: Change
	type to displaced_step_closure_up.
	* rs6000-tdep.c (ppc_displaced_step_copy_insn): Use
	displaced_step_closure_up.
	* s390-tdep.c (s390_displaced_step_copy_insn): Likewise.