Support FixedVector in gdb `idx'

Reviewed By: @alexmalyshev

Differential Revision: D1497321
Fix printing of objects in gdb `idx'

Summary: Changes `idx` to set $# to the address of the indexed value; gdb's
object print format and object expression format are just too different.

Also fixes weirdness in the printing caused by the pretty printer.

Reviewed By: @alexmalyshev

Differential Revision: D1497325
Use print() for python3

Reviewed By: @alexmalyshev

Differential Revision: D1498114