Add API for fetching all persisted compiler diagnostics

[jonatanklosko]

Adds Mix.Task.Compiler.diagnostics() that fetches all warning diagnostics persisted by each compiler in its manifest.

[josevalim]

Fantastic! I think we only need a test for Mix.Task.Compiler.diagnostics and we are good to go. We can probably just add a few lines to one test in compile.elixir and compile.erlang that makes sure it is included there too.

[jonatanklosko]

@josevalim done!

[josevalim]

💚 💙 💜 💛 ❤️