[crashlog] Pass the debugger around instead of relying on lldb.debugger

The lldb.debugger et al convenience variables are only available from
the interactive script interpreter. In all other scenarios, they are
None (since fc1fd6bf) before that they
were default initialized.

The crashlog script was hacking around that by setting the lldb.debugger
to a newly created debugger instance when running outside of the script
interpreter, which works fine until lldb creates a script interpreter
instance under the hood and clears the variables. This was resulting in
an AttributeError when invoking the script directly (from outside of
lldb):

  AttributeError: 'NoneType' object has no attribute 'GetSourceManager'

This patch fixes that by passing around the debugger instance.

rdar://64775776

Differential revision: https://reviews.llvm.org/D90706