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

This is the same idea as the previous patch, but for filtered_iterator.
Without this patch, I would see this when applying the patch that
removes reference_to_pointer_iterator from breakpoint_range:

      CXX    breakpoint.o
    /home/smarchi/src/binutils-gdb/gdb/breakpoint.c: In function ‘void download_tracepoint_locations()’:
    /home/smarchi/src/binutils-gdb/gdb/breakpoint.c:11007:41: error: cannot allocate an object of abstract type ‘breakpoint’
    11007 |   for (breakpoint &b : all_tracepoints ())
          |                                         ^
    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 ()
          | ^~~~~~~~~~

Change-Id: I05285ff27d21cb0ab80cba392ec4e959167e3cd7
Reviewed-By: Andrew Burgess <aburgess@redhat.com>

gdbsupport/filtered-iterator.h

@@ -19,6 +19,8 @@
 #ifndef COMMON_FILTERED_ITERATOR_H
 #define COMMON_FILTERED_ITERATOR_H
 
+#include "gdbsupport/invoke-result.h"
+
 /* A filtered iterator.  This wraps BaseIterator and automatically
    skips elements that FilterFunc filters out.  Requires that
    default-constructing a BaseIterator creates a valid one-past-end
@@ -54,7 +56,10 @@ public:
     : filtered_iterator (static_cast<const filtered_iterator &> (other))
   {}
 
-  value_type operator* () const { return *m_it; }
+  typename gdb::invoke_result<decltype(&BaseIterator::operator*),
+			      BaseIterator>::type
+    operator* () const
+  { return *m_it; }
 
   self_type &operator++ ()
   {