[CSSimplify] Allow type inference through marker protocol existential casts

```swift
class A<T> {
}

class B<U> : A<U> {
}

func test(v: any B<Int> & Sendable) {
  _ = v as A // infers `Int` for `A.T`
}
```

Author: xedin

lib/Sema/CSSimplify.cpp

@@ -9215,6 +9215,23 @@ ConstraintSystem::simplifyCheckedCastConstraint(
       }
     }
 
+    // Peel off marker protocol requirements if this is an existential->concrete
+    // cast. Handles cases like `WritableKeyPath<...> & Sendable as KeyPath`
+    // that require inference which is only attempted if both sides are classes.
+    if (fromType->isExistentialType() && !toType->isExistentialType()) {
+      if (auto *existential = fromType->getAs<ExistentialType>()) {
+        if (auto *PCT = existential->getConstraintType()
+                            ->getAs<ProtocolCompositionType>()) {
+          auto newConstraintTy = PCT->withoutMarkerProtocols();
+          if (!newConstraintTy->isEqual(PCT)) {
+            fromType = newConstraintTy->getClassOrBoundGenericClass()
+                           ? newConstraintTy
+                           : ExistentialType::get(newConstraintTy);
+          }
+        }
+      }
+    }
+
     // We've decomposed the types further, so adopt the subflags.
     flags = subflags;
 

test/Concurrency/sendable_keypaths.swift

@@ -230,3 +230,11 @@ do {
   func forward<T>(_ v: T) -> T { v }
   let _: KeyPath<String, Int> = forward(kp()) // Ok
 }
+
+do {
+  final class C<T> {
+    let immutable: String = ""
+  }
+
+  _ = \C<Int>.immutable as? ReferenceWritableKeyPath // Ok
+}

test/expr/cast/as_coerce.swift

@@ -201,3 +201,16 @@ do {
   (fn as () -> Void)() // expected-error {{no exact matches in reference to local function 'fn'}}
   (fn_1 as () -> Void)() // expected-error {{cannot convert value of type '(Bool) -> ()' to type '() -> Void' in coercion}}
 }
+
+// Test generic parameter inference through casts
+do {
+  class A<T> {
+  }
+
+  class B<U> : A<U> {
+  }
+
+  func test(v: any B<Int> & Sendable) {
+    _ = v as A // infers `Int` for `A.T`
+  }
+}