Instead of using the `isolated P` syntax, switch to specifying the
global actor type directly, e.g.,
class MyClass: @MainActor MyProto { ... }
No functionality change at this point
Allow a conformance to be "isolated", meaning that it stays in the same
isolation domain as the conforming type. Only allow this for
global-actor-isolated types.
When a conformance is isolated, a nonisolated requirement can be
witnessed by a declaration with the same global actor isolation as the
enclosing type.
We had an exploded form of conformance attributes (@unchecked,
@preconcurrency, @unsafe) at several different places in the compiler.
Pull these into a single structure so it's easier to manage and extend.
Should have done this a long time ago.
