[lldb/crashlog] Expand crash report file path before parsing

This patch should fix a crash in the opening a crash report that was
passed with a relative path.

This patch expands the crash report path before parsing it and raises a
`FileNotFoundError` exception if the file doesn't exist.

Differential Revision: https://reviews.llvm.org/D152012



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