relates to:

• SR-9465 [AD] Use a single @differentiable attribute as both a decl attr and a type attr.

Issue Description:

The `@autodiff` attribute was never designed to exist – it existed because the lexer can only lex `@differentiable` to one token kind without extra lookahead. We should fix that.

Both the type attribute and the decl attribute can be followed by a parenthesis and an identifier.
Decl attributes: `@differentiable(wrt: x)`, `@differentiable(where T : FloatingPoint)`
Type attributes: `@differentiable (T) -> U`, `@differentiable SomeTypeAlias`