[AutoDiff] SILGen derivative function thunks must not be transparent.

SILGen generates thunks for derivative functions registered via
`@differentiable` and `@differentiating` attributes.

The thunks have a canonical, consistent naming (currently based on
the original function name and parameter indices) that TBDGen can also
generate via `SILDeclRef::mangle`.

These SILGen derivative function thunks must not be transparent; otherwise,
the functions will be inlined during MandatoryInlining and will not be
exposed publicly.

This bug was discovered as a DeadFunctionElimination crash:
`function_ref` instructions referencing transparent derivative thunks
had their referenced functions set to null by MandatoryInling and became
invalid.

Author: dan-zheng

lib/SILGen/SILGenThunk.cpp

@@ -87,7 +87,7 @@ SILGenModule::getOrCreateAutoDiffThunk(SILDeclRef derivativeFnDeclRef,
       originalLinkage, /*isDerivativeFnExported*/ true);
   auto name = derivativeFnDeclRef.mangle();
   auto *thunk = builder.getOrCreateFunction(
-      derivativeFnDecl, name, linkage, derivativeFnTy, IsBare, IsTransparent,
+      derivativeFnDecl, name, linkage, derivativeFnTy, IsBare, IsNotTransparent,
       derivativeFnDeclRef.isSerialized(), IsNotDynamic, ProfileCounter(),
       IsThunk);
   if (!thunk->empty())

test/AutoDiff/silgen_thunking/main.swift

@@ -10,6 +10,17 @@
 import StdlibUnittest
 import DifferentiationUnittest
 
+// Verify that SILGen derivative thunks are never `[transparent]`.
+@differentiable(vjp: vjpNoReabstraction)
+func noReabstraction<T: Differentiable>(_ x: T) -> T {
+  return x
+}
+func vjpNoReabstraction<T: Differentiable>(_ x: T) -> (T, (T.TangentVector) -> T.TangentVector) {
+  return (x, { $0 })
+}
+// Find the non-`[transparent]` SILGen thunk.
+// CHECK-LABEL: sil hidden [thunk] [ossa] @AD__$s4main15noReabstractionyxxs15_DifferentiableRzlF__vjp_src_0_wrt_0 : $@convention(thin) <τ_0_0 where τ_0_0 : _Differentiable> (@in_guaranteed τ_0_0) -> (@out τ_0_0, @owned @callee_guaranteed (@in_guaranteed τ_0_0.TangentVector) -> @out τ_0_0.TangentVector)
+
 var DerivativeSILGenThunkTests = TestSuite("DerivativeSILGenThunks")
 
 // TF-619: Test cross-module import of `@differentiable` methods with