LoadableByAddress: Fix code that changes lowered return type of function.

It confused the original type's substituted generic signature for the new
type's invocation generic signature, leading to corrupted SIL when a
function had a substituted SIL function type with a return type involving
large loadable types that have to be passed indirectly in IR.

Fixes rdar://87792152

Author: jckarter

lib/IRGen/LoadableByAddress.cpp

@@ -2362,7 +2362,7 @@ static bool rewriteFunctionReturn(StructLoweringState &pass) {
     }
 
     auto NewTy = SILFunctionType::get(
-        loweredTy->getSubstGenericSignature(),
+        loweredTy->getInvocationGenericSignature(),
         loweredTy->getExtInfo(),
         loweredTy->getCoroutineKind(),
         loweredTy->getCalleeConvention(),

test/IRGen/loadable_by_address_subst_function_type_return.swift

@@ -0,0 +1,18 @@
+// rdar://87792152
+// RUN: %target-swift-frontend -disable-availability-checking -emit-ir -verify %s
+
+public struct S1 {
+  var a: Int?
+  var b: Int?
+  var c: Int?
+}
+
+public struct S2 {
+  public func foo() {
+    _ = bar
+  }
+
+  func bar(_: S1) -> some Any {
+    return 0
+  }
+}