Remove pid_to_ptid

This removes pid_to_ptid in favor of calling the ptid_t constructor
directly.

gdb/ChangeLog
2018-07-03  Tom Tromey  <tom@tromey.com>

	* common/ptid.c (pid_to_ptid): Remove.
	* common/ptid.h (pid_to_ptid): Don't declare.
	* aix-thread.c: Update.
	* arm-linux-nat.c: Update.
	* common/ptid.c: Update.
	* common/ptid.h: Update.
	* corelow.c: Update.
	* ctf.c: Update.
	* darwin-nat.c: Update.
	* fbsd-nat.c: Update.
	* fork-child.c: Update.
	* gnu-nat.c: Update.
	* go32-nat.c: Update.
	* inf-ptrace.c: Update.
	* infcmd.c: Update.
	* inferior.c: Update.
	* infrun.c: Update.
	* linux-fork.c: Update.
	* linux-nat.c: Update.
	* nat/aarch64-linux-hw-point.c: Update.
	* nat/fork-inferior.c: Update.
	* nat/x86-linux-dregs.c: Update.
	* nto-procfs.c: Update.
	* obsd-nat.c: Update.
	* procfs.c: Update.
	* progspace.c: Update.
	* remote.c: Update.
	* rs6000-nat.c: Update.
	* s390-linux-nat.c: Update.
	* sol-thread.c: Update.
	* spu-linux-nat.c: Update.
	* target.c: Update.
	* top.c: Update.
	* tracefile-tfile.c: Update.
	* windows-nat.c: Update.

gdb/gdbserver/ChangeLog
2018-07-03  Tom Tromey  <tom@tromey.com>

	* linux-low.c: Update.
	* linux-ppc-low.c: Update.
	* linux-x86-low.c: Update.
	* proc-service.c: Update.
	* server.c: Update.
	* spu-low.c: Update.
	* thread-db.c: Update.
	* win32-low.c: Update.

gdb/target.c

@@ -3242,7 +3242,7 @@ target_announce_detach (int from_tty)
 
   pid = ptid_get_pid (inferior_ptid);
   printf_unfiltered (_("Detaching from program: %s, %s\n"), exec_file,
-		     target_pid_to_str (pid_to_ptid (pid)));
+		     target_pid_to_str (ptid_t (pid)));
   gdb_flush (gdb_stdout);
 }