gdbsupport: make basic_safe_iterator::operator* return the same thing as underlying iterator

Using the following patch that removes the reference_to_pointer_iterator
from breakpoint_range, I would get:

      CXX    breakpoint.o
    /home/smarchi/src/binutils-gdb/gdb/breakpoint.c: In function ‘void breakpoint_program_space_exit(program_space*)’:
    /home/smarchi/src/binutils-gdb/gdb/breakpoint.c:3030:46: error: cannot allocate an object of abstract type ‘breakpoint’
     3030 |   for (breakpoint &b : all_breakpoints_safe ())
          |                                              ^
    In file included from /home/smarchi/src/binutils-gdb/gdb/gdbthread.h:26,
                     from /home/smarchi/src/binutils-gdb/gdb/infrun.h:21,
                     from /home/smarchi/src/binutils-gdb/gdb/gdbarch.h:28,
                     from /home/smarchi/src/binutils-gdb/gdb/arch-utils.h:23,
                     from /home/smarchi/src/binutils-gdb/gdb/breakpoint.c:21:
    /home/smarchi/src/binutils-gdb/gdb/breakpoint.h:619:8: note:   because the following virtual functions are pure within ‘breakpoint’:
      619 | struct breakpoint : public intrusive_list_node<breakpoint>
          |        ^~~~~~~~~~
    /home/smarchi/src/binutils-gdb/gdb/breakpoint.c:250:1: note:     ‘virtual breakpoint::~breakpoint()’
      250 | breakpoint::~breakpoint ()
          | ^~~~~~~~~~

This is because the operator* method of the basic_safe_iterator iterator
wrapper returns a value_type.  So, even if the method of the underlying
iterator (breakpoint_iterator, an intrusive_list iterator) returns a
`breakpoint &`, the method of the wrapper returns a `breakpoint`.

I think it would make sense for iterator wrappers such as
basic_safe_iterator to return the exact same thing as the iterator they
wrap.  At least, it fixes my problem.

Change-Id: Ibbcd390ac03d2fb6ae4854923750c8d7c3c04e8a
Reviewed-By: Andrew Burgess <aburgess@redhat.com>

gdbsupport/safe-iterator.h

@@ -19,6 +19,8 @@
 #ifndef COMMON_SAFE_ITERATOR_H
 #define COMMON_SAFE_ITERATOR_H
 
+#include "gdbsupport/invoke-result.h"
+
 /* A forward iterator that wraps Iterator, such that when iterating
    with iterator IT, it is possible to delete *IT without invalidating
    IT.  Suitably wrapped in a range type and used with range-for, this
@@ -75,7 +77,9 @@ public:
   basic_safe_iterator ()
   {}
 
-  value_type operator* () const { return *m_it; }
+  typename gdb::invoke_result<decltype(&Iterator::operator*), Iterator>::type
+    operator* () const
+  { return *m_it; }
 
   self_type &operator++ ()
   {