[display] use a separate diagnostics kind for deprecation warnings

[Gama11]

This lets us add support for DiagnosticTag.Deprecated in vshaxe (without this, we can't really tell deprecation warnings apart from other warnings).

[Simn]

We should check if everything related to deprecation warnings goes through this, but the change is good regardless.

[Gama11]

Hm.. I was about to say "where else would it go", but I guess there's at least one "custom" deprecation warning.. :/

haxe/src/typing/typeload.ml

Lines 253 to 257 in 710eea6

	let msg = match Meta.get Meta.Deprecated td.t_meta with
	| _,[EConst(String s),_],_ -> s
	| _ -> "This typedef is deprecated in favor of " ^ (s_type (print_context()) td.t_type)
	in
	ctx.com.warning msg p

[Simn]

IIRC we had to check that early because the typedef would disappear in some cases.