[AutoDiff] NFC: improve debug log output.

[dan-zheng]

Print value/instruction with context in non-differentiability error debug log.

---

Before: the source of some non-differentiability issues isn't clear.

$ swiftc test.swift
...
[AD] Diagnosing non-differentiability.
[AD] For value:
  %10 = partial_apply [callee_guaranteed] %9<τ_0_0, τ_1_0>() : $@convention(thin) <τ_0_0, τ_0_1> (@in_guaranteed τ_0_0, @guaranteed KeyPath<τ_0_0, τ_0_1>) -> @out τ_0_1 // user: %11
[AD] With invoker:
(differentiation_invoker differentiable_function_inst=(  %11 = differentiable_function [parameters 0] [results 0] %10 : $@callee_guaranteed (@in_guaranteed τ_0_0, @guaranteed KeyPath<τ_0_0, τ_1_0>) -> @out τ_1_0 // users: %16, %12
))

After:

$ swiftc test.swift
...
[AD] Diagnosing non-differentiability.
[AD] For value:
     // function_ref swift_getAtKeyPath
  %9 = function_ref @swift_getAtKeyPath : $@convention(thin) <τ_0_0, τ_0_1> (@in_guaranteed τ_0_0, @guaranteed KeyPath<τ_0_0, τ_0_1>) -> @out τ_0_1 // user: %10
->   %10 = partial_apply [callee_guaranteed] %9<τ_0_0, τ_1_0>() : $@convention(thin) <τ_0_0, τ_0_1> (@in_guaranteed τ_0_0, @guaranteed KeyPath<τ_0_0, τ_0_1>) -> @out τ_0_1 // user: %11
     %11 = differentiable_function [parameters 0] [results 0] %10 : $@callee_guaranteed (@in_guaranteed τ_0_0, @guaranteed KeyPath<τ_0_0, τ_1_0>) -> @out τ_1_0 // users: %16, %12
[AD] With invoker:
(differentiation_invoker differentiable_function_inst=(  %11 = differentiable_function [parameters 0] [results 0] %10 : $@callee_guaranteed (@in_guaranteed τ_0_0, @guaranteed KeyPath<τ_0_0, τ_1_0>) -> @out τ_1_0 // users: %16, %12
))

[dan-zheng]

@swift-ci Please smoke test

[dan-zheng]

@swift-ci Please smoke test macOS