Added support of info messages

[dmatousek]

The PR adds support of info messages. The behavior is the same as with errors and warnings.

An example of a call is the following:

::info(ErrorType::INFO_INFERRED, "%1%: default value inferred", node);

With the following output:

main.p4(34): [--Winfo=inferred] info: start: default value inferred
    state start {
          ^^^^^

A new command-line option --Winfo=diagnostic is introduced. It allows to demote warnings to info messages (e.g., --Winfo=unused). --Wwarn has a new meaning. It promotes all info messages to warnings (it mimics --Werror).