[lldb/crashlog] Add support for Last Exception Backtrace

This patch adds support to the "Last Exception Backtrace" to the
`crashlog` command.

This metadata is homologous to the "Application Specific Backtrace",
however the format is closer to a regular stack frame.

Since the thread that "contains" the "Last Exception Backtrace" doesn't
really exist, this information is displayed when requesting an extended
backtrace of the crashed thread, similarly to the "Application Specific
Backtrace".

To achieve that, this patch includes some refactors and fixes to the
existing "Application Specific Backtrace" handling.

rdar://113046509

Differential Revision: https://reviews.llvm.org/D157851



Signed-off-by: Med Ismail Bennani <ismail@bennani.ma>