Correctly filter Flymake diagnostic types shown in mode-line

Correctly filter Flymake diagnostic types shown in mode-line

Thus, if a package foo has its own types foo-error and
foo-warning, and if the buffer has no errors, the mode-line
will correctly show `[0 0]' (zero errors and warnings) instead
of `[0 0 0 0]' (zero errors, zero foo-errors, zero warnings,
zero  foo-warnings).

* lisp/progmodes/flymake.el
(flymake--mode-line-format): Coalesce diagnostic types based on
the severity, not the symbol.