The attribute is now allowed on an assortment of declarations, to
suppress warnings related to declarations themselves, or all warnings in
the lexical scope of the declaration.
I don't necessarily see a reason to have a list at all, but it does look
as if some of those more niche items aren't properly supported by the
compiler itself so let's maintain a short safe list for now.
The initial implementation raised a question whether the attribute
should apply to lexical declaration context vs. "actual" declaration
context. I'm using "lexical" here because it results in less warnings
suppressed, which is the conservative behavior: we can always expand it
later if we think this is wrong, without breaking any existing code. I
also think that this is the correct behavior that we will probably never
want to change, given that the user typically desires to keep the
suppressions as localized as possible.
