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/linux-fork.c

@@ -216,7 +216,7 @@ fork_id_to_ptid (int num)
   if (fork)
     return fork->ptid;
   else
-    return pid_to_ptid (-1);
+    return ptid_t (-1);
 }
 
 static void