Use working debugger for debuginfo tests

[Noratrieb]

Currently, bootstrap/compiletest use a system debugger (like gdb or lldb) for debuginfo tests. People have very different setups and sometimes their system debugger simply doesn't work for the tests.

We shouldn't be using random system tools for the test suite unless we know that they work.

The debuginfo tests should just always download and use one that works. This needs some figuring out what the right one for that would be and where to get it from (do we have LLDB builds in our CI?).

[saethlin]

Note that this is not just the case for broken debuggers (like the most recent gdb release) but also for debuggers with snazzy new features (such as a previous gdb release): #94458