SILOptimizer: fix partial_apply optimization.

[dan-zheng]

partial_apply can be rewritten to thin_to_thick_function only if the
specialized callee is @convention(thin).

This condition is newly exercised by the differentiation transform:
{JVP,VJP}Emitter::visitApplyInst generates argument-less partial_apply
with @convention(method) callees.

Re-enables tests disabled in #31545. CI breakages should be fixed.

Resolves SR-12732:

SIL verification failed: operand of thin_to_thick_function must be thin: opFTy->getRepresentation() == SILFunctionType::Representation::Thin
Verifying instruction:
     // function_ref specialized Differentiable._vjpWithDerivative(_:)
  %10 = function_ref @$s16_Differentiation14DifferentiablePAAE18_vjpWithDerivativeyx5value_13TangentVectorQzAGc8pullbacktyAGzcF0A8Unittest7TrackedVySfG_Tg5 : $@convention(method) (@guaranteed @callee_guaranteed @substituted <τ_0_0> (@inout τ_0_0) -> () for <Tracked<Float>>, @in_guaranteed Tracked<Float>) -> (@out Tracked<Float>, @owned @callee_guaranteed @substituted <τ_0_0, τ_0_1> (@in_guaranteed τ_0_0) -> @out τ_0_1 for <Tracked<Float>, Tracked<Float>>) // user: %11
->   %11 = thin_to_thick_function %10 : $@convention(method) (@guaranteed @callee_guaranteed @substituted <τ_0_0> (@inout τ_0_0) -> () for <Tracked<Float>>, @in_guaranteed Tracked<Float>) -> (@out Tracked<Float>, @owned @callee_guaranteed @substituted <τ_0_0, τ_0_1> (@in_guaranteed τ_0_0) -> @out τ_0_1 for <Tracked<Float>, Tracked<Float>>) to $@callee_guaranteed (@guaranteed @callee_guaranteed @substituted <τ_0_0> (@inout τ_0_0) -> () for <Tracked<Float>>, @in_guaranteed Tracked<Float>) -> (@out Tracked<Float>, @owned @callee_guaranteed @substituted <τ_0_0, τ_0_1> (@in_guaranteed τ_0_0) -> @out τ_0_1 for <Tracked<Float>, Tracked<Float>>) // user: %12

[dan-zheng]

It would be nice to remove the dependency on DifferentiationUnittest. I tried but didn't quite figure it out: -enable-library-evolution seems involved, but passing it exposes another bug (TF-1274).

Let's visit this later to unblock progress.

[dan-zheng]

@swift-ci Please test

[dan-zheng]

Merging to fix CI breakages. Happy to address any feedback later!