[6.2][Concurrency] Correct the signing of priority escalation handler

[ktoso]

Description:

In #81229 we used the wrong discriminator number: we used the witness table discriminator for the TaskPriority:

	mov	x17, x8
	movk	x17, #11839, lsl #48 <<<<< bad
	autda	x16, x17

while we should have used the one for the branch itself:

	stur	x1, [x29, #-32]
	bl	static Swift.TaskPriority.default.getter : Swift.TaskPriority
	mov	x0, x20
	bl	_swift_retain
	ldur	x8, [x29, #-88]
	ldur	x1, [x29, #-56]
	mov	x0, x1
	mov	x17, #3955 <<<<<< good
	blraa	x8, x17

This "actually" resolves rdar://150378890 which #81229 was intending to do

Scope/Impact: Only fixes the value used for pointer signing
Risk: Low; without change this API never worked before on arm64e.
Testing: Manually verified this is the correct signing
Reviewed by: @mikeash

Original PR: #81755
Radar: rdar://150378890

[ktoso]

@swift-ci please test