Read XML into attributes.

[stephen-hawley]

Added an AttributeDeclaration type in order to round-trip attributes.

An attribute in swift is simultaneously overly complicated and inflexible.
The syntax is an @ sign followed by a name followed by an optional argument set off by parens.
What can go in the parens is a seamy, inconsistent set of junk that can included nested lists set off by parentheses, brackets, or braces (as long as they balance).
Examples:

@foo
@available(swift *)
@available(iOS 14.*, unavailable, macOS *, available)
@someotherthing(foo: bar, baz: [15 .. 20], "a string because why not")

And the contents of the parentheses is determined entirely by the name of the attribute.

So how should we represent this in C#? We get an AttributeDeclaration which has a name and an optional list of parameters.
A parameter is one of a plain AttributeParameter (should never happen), an AttributeParameterLabel (any non-number), an AttributeParameterLiteral (a number), an AttributeParameterSublist.

The XML consumption code is straightforward, although I'm sure it could be better where the right set of attributes/names would make it just read in on its own (cue "old man shouts at clouds" meme).

I added a List to the BaseDeclaration and therefore attributes are now available on classes, structs, enums, protocols, properties, and functions (take that, anti-inheritance people!).

In addition, the original grammar definition tried to break out a separate rule for platform and version, but it was broken and couldn't handle wild cards, so I got rid of it entirely and got rid of the version parameter. I also made separators and operators get treated as labels. There really isn't any advantage to having those broken out right now (if ever).