Add is_implicit_ptr_at method to dwarf_location

Another expectation of the existing function callback interface of the
computed struct value is to check if a specific part (on a given offset
of a given length) of an underlying location description is an implicit
pointer location description.

To satisfy this expectation a new is_implicit_ptr_at has been added.

gdb/ChangeLog:

        * dwarf2/expr.c (dwarf_location::is_implicit_ptr_at):
        New method.
        (dwarf_implicit_pointer::is_implicit_ptr_at): New method.
        (dwarf_composite::is_implicit_ptr_at): New method.

gdb/dwarf2/expr.c

@@ -457,6 +457,14 @@ public:
 				   LONGEST bits_to_skip, size_t bit_size,
 				   size_t location_bit_limit) const;
 
+  /* Check if a given DWARF location description contains an implicit
+     pointer location description of a BIT_LENGTH size on a given
+     BIT_OFFSET offset.  */
+  virtual bool is_implicit_ptr_at (LONGEST bit_offset, int bit_length) const
+  {
+     return false;
+  }
+
 protected:
   /* Architecture of the location.  */
   gdbarch *m_arch;
@@ -1088,6 +1096,11 @@ public:
 			   size_t location_bit_limit) const override
   {}
 
+  bool is_implicit_ptr_at (LONGEST bit_offset, int bit_length) const override
+  {
+     return true;
+  }
+
 private:
   /* Per object file data of the implicit pointer.  */
   dwarf2_per_objfile *m_per_objfile;
@@ -1174,6 +1187,8 @@ public:
 			   LONGEST bits_to_skip, size_t bit_size,
 			   size_t location_bit_limit) const override;
 
+  bool is_implicit_ptr_at (LONGEST bit_offset, int bit_length) const override;
+
 private:
   /* Composite piece that contains a piece location
      description and it's size.  */
@@ -1370,6 +1385,43 @@ dwarf_composite::write_to_gdb_value (frame_info *frame, struct value *value,
     }
 }
 
+bool
+dwarf_composite::is_implicit_ptr_at (LONGEST bit_offset, int bit_length) const
+{
+  /* Advance to the first non-skipped piece.  */
+  unsigned int pieces_num = m_pieces.size ();
+  LONGEST total_bit_offset = bit_offset;
+  LONGEST total_bit_length = bit_length;
+
+  total_bit_offset += HOST_CHAR_BIT * m_offset + m_bit_suboffset;
+
+  for (unsigned int i = 0; i < pieces_num && total_bit_length != 0; i++)
+    {
+      const piece &piece = m_pieces[i];
+      ULONGEST read_bit_length = piece.size;
+
+      if (total_bit_offset >= read_bit_length)
+	{
+	  total_bit_offset -= read_bit_length;
+	  continue;
+	}
+
+      read_bit_length -= total_bit_offset;
+
+      if (total_bit_length < read_bit_length)
+	read_bit_length = total_bit_length;
+
+      if (piece.location->is_implicit_ptr_at (total_bit_offset,
+					      read_bit_length))
+	return true;
+
+      total_bit_offset = 0;
+      total_bit_length -= read_bit_length;
+    }
+
+    return false;
+}
+
 struct piece_closure
 {
   /* Reference count.  */