[Attributor] Support captures attribute

[nikic]

Attributor can currently only infer captures(none). It should be switched to infer the full CaptureInfo, similar to what FunctionAttrs does. The custom "no-capture-maybe-returned" attribute should be subsumed by this as well.

[antoniofrighetto]

If I'm understanding a bit of Attributor code correctly, there are quite a number of structs method to move to infer the full CaptureInfo; for instance, AANoCaptureArgument for argument-specific handling, AANoCaptureCallSiteArgument for call-site arguments, possibly AANoCaptureReturned too which right now is not handled at all... However, I think the primary changes should boil down to the bits used to implement the abstract state components. Particularly, the whole AANoCapture state encoding enum (and its uses):

/// State encoding bits.
enum {
  NOT_CAPTURED_IN_MEM = 1 << 0,
  NOT_CAPTURED_IN_INT = 1 << 1,
  NOT_CAPTURED_IN_RET = 1 << 2,
  NO_CAPTURE_MAYBE_RETURNED = NOT_CAPTURED_IN_MEM | NOT_CAPTURED_IN_INT,
  NO_CAPTURE = NOT_CAPTURED_IN_MEM | NOT_CAPTURED_IN_INT | NOT_CAPTURED_IN_RET,
};

I think this whole enum should be replaced with CaptureInfo-based state. Afterwards, the various implementation methods of AANoCapture/AANoCaptureImpl using such an enum should be reworked too. @nikic As approximate as this may sound, does this intuitively seem coherent to you?