[lldb] Add Checksum to FileSpec (#71457)

Store a Checksum in FileSpec. Its purpose is to store the MD5 hash that
was added to the DWARF 5 line table.

This increases the size of a FileSpec from 24 to 40 bytes. The
alternative is to introduce a new SupportFile abstraction for a FileSpec
+ Checksum but that would require a corresponding SupportFileList class.
During review we decided that wasn't worth it, but that's something we
can revisit in the future.