[pushed/Ada] Crash with references to GNAT packed arrays handling

[Joel Brobecker]

Hello,

I have just pushed this patch series consisting in one fix from
Pierre-Marie de Rodat, and a testcase that I wrote for it.

This situation showed up on our radar as we are working towards
reducing the use of our GNAT encodings for renamings, trying to
replace them with a pure DWARF description instead.

Tested on x86_64-linux.