[lldb] Move MD5 Checksum from FileSpec to SupportFile

When I added the MD5 checksum I was on the fence between storing it in
FileSpec or creating a new SupportFile abstraction. The latter was
deemed overkill for just the MD5 hashes, but support for inline sources
in the DWARF 5 line table tipped the scales. This patch moves the MD5
checksum into the new SupportFile class.