Fix tensorflow/swift-apis compilation.

[dan-zheng]

Remove problematic call to TC.conformsToProtocol on synthesized
AllDifferentiableVariables struct, which produced an ErrorType
and caused an IRGen assertion failure.

tensorflow/swift-apis now compiles.

[dan-zheng]

Testing something - please hold off from merging.

[dan-zheng]

I tried moving the call to TC.conformsToProtocol(allDiffableVarsStruct->getDeclaredTypeInContext(), ...) below, after adding typealiases to allDiffableVarsStruct:

diff --git a/lib/Sema/DerivedConformanceDifferentiable.cpp b/lib/Sema/DerivedConformanceDifferentiable.cpp
index 9c43d03eb1..02d425d9c2 100644
--- a/lib/Sema/DerivedConformanceDifferentiable.cpp
+++ b/lib/Sema/DerivedConformanceDifferentiable.cpp
@@ -1130,6 +1130,12 @@ deriveDifferentiable_AssociatedStruct(DerivedConformance &derived,
         allDiffableVarsStruct, allDiffableVarsStruct, TC);
     addAssociatedTypeAliasDecl(C.Id_CotangentVector,
         allDiffableVarsStruct, allDiffableVarsStruct, TC);
+    // `AllDifferentiableVariables` must conform to `AdditiveArithmetic`.
+    // This should be guaranteed.
+    assert(TC.conformsToProtocol(
+               allDiffableVarsStruct->getDeclaredTypeInContext(), addArithProto,
+               parentDC, ConformanceCheckFlags::Used) &&
+           "`AllDifferentiableVariables` must conform to `AdditiveArithmetic`");
     addAssociatedTypeAliasDecl(C.Id_TangentVector,
         parentDC, allDiffableVarsStruct, TC);
     addAssociatedTypeAliasDecl(C.Id_CotangentVector,

I thought this might help fix the crash, but it didn't.