[analyzer] Add explanation why analyzer report is not generated (fix for PR12421).

Currently if the path diagnostic consumer (e.g HTMLDiagnostics and PlistDiagnostics) do not support cross file diagnostics then the path diagnostic report is silently omitted in the case of cross file diagnostics. The patch adds a little verbosity to Clang in this case.
The patch also adds help entry for the "--analyzer-output" driver option.

llvm-svn: 283499