Introduce a crate for common AST definitions

[Centril]

Context: #65884 (comment)

For example:

• Mutability is shared between AST / HIR / ...
• CaptureBy and CaptureClause fill the same role.
• Constness
• Unsafety
• Movability and GeneratorMovability (?)
• BindingMode

Suggested crate name rustc_ast_common.

cc @oli-obk

[cjgillot]

Hi,

I would like to do that if you don't mind.
I will try to submit a draft during the weekend.

[oli-obk]

wrt to creating a global crate, we should hold up on that. Merging types where no new crate is needed is uncontroversial I believe, but adding a new crate just for this should be discussed first

[Centril]

Closing as moot for the time being.