test: loosen the differentiability witness test

There is currently a difference between the tensorflow branch and the
master branch.  On tensorflow, the differentiability support is merged
into the standard library.  This changes the decoration of the witness.
Loosen the test to accept either.

We should change the tensorflow branch to generate the
`_Differentiation` module with the support and then auto-import the
module in the longer term.  This can be gated by the
`-enable-experimental-autodifferentiation` flag to the driver to gain
the same behaviour on both the branches.

Author: compnerd

test/AutoDiff/SIL/Serialization/sil_differentiability_witness.sil

@@ -161,7 +161,7 @@ sil_differentiability_witness hidden [parameters 0 1] [results 0] <τ_0_0 where
 // ROUNDTRIP:   vjp: @AD__generic__vjp_src_0_wrt_0_1 : $@convention(thin) <τ_0_0 where τ_0_0 : Differentiable> (@in_guaranteed τ_0_0, Float) -> (@out τ_0_0, @owned @callee_guaranteed (@in_guaranteed τ_0_0.TangentVector) -> (@out τ_0_0.TangentVector, Float))
 // ROUNDTRIP: }
 
-// IRGEN-LABEL: @AD__generic_PSSRS16_Differentiation14DifferentiableRzl = hidden global { i8*, i8* } {
+// IRGEN: @AD__generic_PSSRS{{16_Differentiation|s}}14DifferentiableRzl = hidden global { i8*, i8* } {
 // IRGEN-SAME: @AD__generic__jvp_src_0_wrt_0_1
 // IRGEN-SAME: @AD__generic__vjp_src_0_wrt_0_1
 // IRGEN-SAME: }