FunctionSignatureOpts: fix a crash due to a missing argument declaration

The existential -> generic transformation in FSO didn't preserve the argument declaration.
This caused an assert to hit in a later FSO transformation.

rdar://problem/61206439
https://bugs.swift.org/browse/SR-12487

Author: eeckstein

lib/SILOptimizer/FunctionSignatureTransforms/ExistentialTransform.cpp

@@ -179,7 +179,8 @@ void ExistentialSpecializerCloner::cloneArguments(
         NewF.getLoweredType(NewF.mapTypeIntoContext(GenericParam));
     GenericSILType = GenericSILType.getCategoryType(
                                           ArgDesc.Arg->getType().getCategory());
-    auto *NewArg = ClonedEntryBB->createFunctionArgument(GenericSILType);
+    auto *NewArg =
+      ClonedEntryBB->createFunctionArgument(GenericSILType, ArgDesc.Decl);
     NewArg->setOwnershipKind(ValueOwnershipKind(
         NewF, GenericSILType, ArgDesc.Arg->getArgumentConvention()));
     // Determine the Conformances.

test/SILOptimizer/functionsigopts_crash.swift

@@ -0,0 +1,36 @@
+// RUN: %target-swift-frontend -module-name main -O -emit-sil -primary-file %s | %FileCheck %s
+
+protocol P {
+  func foo()
+}
+
+public struct Inner {
+  var x: Int
+  var y: Int
+}
+
+public struct S : P {
+  var i: Inner
+
+  func foo() {
+    print(i.x)
+  }
+}
+
+// Check that FSO does not crash due to a missing decl on the function argument.
+
+// Following specializations should be done:
+//   * FSO: existential to protocol constrained generic
+//   * generic specialization <S>
+//   * FSO: argument explosion 
+
+// CHECK-LABEL: sil shared [noinline] @$s4main6testityyAA1P_pFTf4e_nAA1SV_Tg5Tf4x_n : $@convention(thin) (Int) -> () { 
+// CHECK-NEXT: // %0 "p"
+@inline(never)
+func testit(_ p: P) {
+  p.foo()
+}
+
+public func callit(s: S) {
+  testit(s)
+}