[AutoDiff] Fix ownership error in VJPCloner::visitApplyInst. (swiftlang#35003)

Change the following code pattern:

```
%x1 = convert_function %x0 : $@differentiable (...) -> ...
%x2 = begin_borrow %x1
... // use %x2
%x3 = end_borrow %x2
destroy_value %x0
```

To the following:

```
%x1 = begin_borrow %x0 : $@differentiable (...) -> ...
%x2 = convert_function %x0
... // use %x2
%x3 = end_borrow %x1
destroy_value %x0
```

Resolves SR-13933: "multiple consuming users" ownership error caused by
`VJPCloner::visitApply` related to `@differentiable`-function-typed callees.

Also upstream test/AutoDiff/SILOptimizer/generics.swift from tensorflow branch.

Author: dan-zheng

lib/SILOptimizer/Differentiation/VJPCloner.cpp

@@ -488,15 +488,17 @@ class VJPCloner::Implementation final
           return;
         }
       }
-      auto origFnType = origCallee->getType().castTo<SILFunctionType>();
-      auto origFnUnsubstType = origFnType->getUnsubstitutedType(getModule());
-      if (origFnType != origFnUnsubstType) {
-        origCallee = builder.createConvertFunction(
-            loc, origCallee, SILType::getPrimitiveObjectType(origFnUnsubstType),
-            /*withoutActuallyEscaping*/ false);
-      }
       builder.emitScopedBorrowOperation(
           loc, origCallee, [&](SILValue borrowedDiffFunc) {
+            auto origFnType = origCallee->getType().castTo<SILFunctionType>();
+            auto origFnUnsubstType =
+                origFnType->getUnsubstitutedType(getModule());
+            if (origFnType != origFnUnsubstType) {
+              borrowedDiffFunc = builder.createConvertFunction(
+                  loc, borrowedDiffFunc,
+                  SILType::getPrimitiveObjectType(origFnUnsubstType),
+                  /*withoutActuallyEscaping*/ false);
+            }
             vjpValue = builder.createDifferentiableFunctionExtract(
                 loc, NormalDifferentiableFunctionTypeComponent::VJP,
                 borrowedDiffFunc);