Fix linkage for references to async function pointers.

We were not making references to async function pointers "weak" when
the function itself was weak, because we were always calculating
linkage as-if we were defining the async function pointer.

Fixes the rest of rdar://79674106.

Author: DougGregor

lib/IRGen/Linking.cpp

@@ -758,7 +758,7 @@ SILLinkage LinkEntity::getLinkage(ForDefinition_t forDefinition) const {
   case Kind::DispatchThunkAllocatorAsyncFunctionPointer:
   case Kind::PartialApplyForwarderAsyncFunctionPointer:
     return getUnderlyingEntityForAsyncFunctionPointer()
-        .getLinkage(ForDefinition);
+        .getLinkage(forDefinition);
   case Kind::KnownAsyncFunctionPointer:
     return SILLinkage::PublicExternal;
   case Kind::PartialApplyForwarder:

test/IRGen/async/weak_availability.swift

@@ -0,0 +1,13 @@
+// RUN: %target-swift-frontend -enable-implicit-dynamic -target %target-cpu-apple-macosx11 -Onone -emit-ir %s | %FileCheck --check-prefix=MAYBE-AVAILABLE %s
+// REQUIRES: OS=macosx && CPU=x86_64
+
+@available(macOS 12.0, *)
+public func f<S: AsyncSequence>(_ s: S) async throws -> Any.Type {
+  for try await _ in s { }
+
+  typealias Fn = @MainActor () -> S.Element
+  return Fn.self
+}
+
+// MAYBE-AVAILABLE: @"$sScI4next7ElementQzSgyYaKFTjTu" = extern_weak global
+// MAYBE-AVAILABLE: declare{{.*}} extern_weak{{.*}} @swift_getFunctionTypeMetadataGlobalActor