gdb: add string_file::release method

A common pattern for string_file is to want to move out the internal
string buffer, because it is the result of the computation that we want
to return.  It is the reason why string_file::string returns a non-const
reference, as explained in the comment.  I think it would make sense to
have a dedicated method for that instead and make string_file::string
return a const reference.

This allows removing the explicit std::move in the typical case.  Note
that compile_program::compute was missing a move, meaning that the
resulting string was copied.  With the new version, it's not possible to
forget to move.

Change-Id: Ieaefa35b73daa7930b2f3a26988b6e3b4121bb79

gdb/ada-varobj.c

@@ -82,7 +82,7 @@ ada_varobj_scalar_image (struct type *type, LONGEST val)
   string_file buf;
 
   ada_print_scalar (type, val, &buf);
-  return std::move (buf.string ());
+  return buf.release ();
 }
 
 /* Assuming that the (PARENT_VALUE, PARENT_TYPE) pair designates
@@ -817,7 +817,7 @@ ada_varobj_get_value_image (struct value *value,
   string_file buffer;
 
   common_val_print (value, &buffer, 0, opts, current_language);
-  return std::move (buffer.string ());
+  return buffer.release ();
 }
 
 /* Assuming that the (VALUE, TYPE) pair designates an array varobj,