[Typed throws] Check thrown error type from an async for..in loop

While it is not yet possible for an `AsyncSequence` to specify its
thrown error type (so it always throws `any Error`), check that `any
Error` is a suitable error type to be thrown in context.

Author: DougGregor

lib/Sema/TypeCheckEffects.cpp

@@ -918,12 +918,15 @@ class ApplyClassifier {
 
   Classification classifyConformance(ProtocolConformanceRef conformanceRef,
                                      EffectKind kind) {
+    if (conformanceRef.isInvalid())
+      return Classification::forInvalidCode();
+
     if (conformanceRef.hasEffect(kind)) {
       assert(kind == EffectKind::Throws); // there is no async
       ASTContext &ctx = conformanceRef.getRequirement()->getASTContext();
       // FIXME: typed throws, if it becomes a thing for conformances
       return Classification::forThrows(
-          ctx.getAnyExistentialType(),
+          ctx.getErrorExistentialType(),
           ConditionalEffectKind::Conditional,
           PotentialEffectReason::forConformance());
     }

test/Concurrency/typed_throws.swift

@@ -0,0 +1,16 @@
+// RUN: %target-typecheck-verify-swift -enable-experimental-feature TypedThrows
+
+// REQUIRES: concurrency
+
+enum MyError: Error {
+  case failed
+  case epicFailed
+}
+
+
+@available(SwiftStdlib 5.1, *)
+func testAsyncFor<S: AsyncSequence>(seq: S) async throws(MyError) {
+  // expected-error@+1{{thrown expression type 'any Error' cannot be converted to error type 'MyError'}}
+  for try await _ in seq {
+  }
+}