Add support for reason on [Constant] attribute.

[TimothyJCowen]

As @j3parker said at https://github.com/Brightspace/D2L.CodeStyle/pull/687/files#r555181048:

Having part of this be customizable @ the parameter defn would be neat. Like why does it need to be constant? etc.
Not in this PR, could just open an issue for later.

public static readonly DiagnosticDescriptor NonConstantPassedToConstantParameter = new DiagnosticDescriptor(
	id: "D2L0074",
	title: "Constant parameter cannot be passed a non-constant value.",
	messageFormat: "Parameter \"{0}\" is marked with the [Constant] attribute, and so cannot be passed a non-constant value.",
	category: "Safety",
	defaultSeverity: DiagnosticSeverity.Error,
	isEnabledByDefault: true,
	description: "The method being called has declared that this parameter must receive a constant, but a non-constant value is being passed."
);