gdb: pass frames as const frame_info_ptr &

We currently pass frames to function by value, as `frame_info_ptr`.
This is somewhat expensive:

 - the size of `frame_info_ptr` is 64 bytes, which is a bit big to pass
   by value
 - the constructors and destructor link/unlink the object in the global
   `frame_info_ptr::frame_list` list.  This is an `intrusive_list`, so
   it's not so bad: it's just assigning a few points, there's no memory
   allocation as if it was `std::list`, but still it's useless to do
   that over and over.

As suggested by Tom Tromey, change many function signatures to accept
`const frame_info_ptr &` instead of `frame_info_ptr`.

Some functions reassign their `frame_info_ptr` parameter, like:

  void
  the_func (frame_info_ptr frame)
  {
    for (; frame != nullptr; frame = get_prev_frame (frame))
      {
        ...
      }
  }

I wondered what to do about them, do I leave them as-is or change them
(and need to introduce a separate local variable that can be
re-assigned).  I opted for the later for consistency.  It might not be
clear why some functions take `const frame_info_ptr &` while others take
`frame_info_ptr`.  Also, if a function took a `frame_info_ptr` because
it did re-assign its parameter, I doubt that we would think to change it
to `const frame_info_ptr &` should the implementation change such that
it doesn't need to take `frame_info_ptr` anymore.  It seems better to
have a simple rule and apply it everywhere.

Change-Id: I59d10addef687d157f82ccf4d54f5dde9a963fd0
Approved-By: Andrew Burgess <aburgess@redhat.com>

gdb/arm-linux-tdep.c

@@ -277,7 +277,7 @@ static struct arm_get_next_pcs_ops arm_linux_get_next_pcs_ops = {
 };
 
 static void
-arm_linux_sigtramp_cache (frame_info_ptr this_frame,
+arm_linux_sigtramp_cache (const frame_info_ptr &this_frame,
 			  struct trad_frame_cache *this_cache,
 			  CORE_ADDR func, int regs_offset)
 {
@@ -300,7 +300,7 @@ arm_linux_sigtramp_cache (frame_info_ptr this_frame,
 /* See arm-linux.h for stack layout details.  */
 static void
 arm_linux_sigreturn_init (const struct tramp_frame *self,
-			  frame_info_ptr this_frame,
+			  const frame_info_ptr &this_frame,
 			  struct trad_frame_cache *this_cache,
 			  CORE_ADDR func)
 {
@@ -320,7 +320,7 @@ arm_linux_sigreturn_init (const struct tramp_frame *self,
 
 static void
 arm_linux_rt_sigreturn_init (const struct tramp_frame *self,
-			  frame_info_ptr this_frame,
+			  const frame_info_ptr &this_frame,
 			  struct trad_frame_cache *this_cache,
 			  CORE_ADDR func)
 {
@@ -343,7 +343,7 @@ arm_linux_rt_sigreturn_init (const struct tramp_frame *self,
 
 static void
 arm_linux_restart_syscall_init (const struct tramp_frame *self,
-				frame_info_ptr this_frame,
+				const frame_info_ptr &this_frame,
 				struct trad_frame_cache *this_cache,
 				CORE_ADDR func)
 {
@@ -756,7 +756,7 @@ arm_linux_core_read_description (struct gdbarch *gdbarch,
    will return to ARM or Thumb code.  Return 0 if it is not a
    rt_sigreturn/sigreturn syscall.  */
 static int
-arm_linux_sigreturn_return_addr (frame_info_ptr frame,
+arm_linux_sigreturn_return_addr (const frame_info_ptr &frame,
 				 unsigned long svc_number,
 				 CORE_ADDR *pc, int *is_thumb)
 {
@@ -1720,7 +1720,7 @@ arm_linux_syscall_record (struct regcache *regcache, unsigned long svc_number)
 /* Implement the skip_trampoline_code gdbarch method.  */
 
 static CORE_ADDR
-arm_linux_skip_trampoline_code (frame_info_ptr frame, CORE_ADDR pc)
+arm_linux_skip_trampoline_code (const frame_info_ptr &frame, CORE_ADDR pc)
 {
   CORE_ADDR target_pc = arm_skip_stub (frame, pc);