[TBDGen] Use default kind for AFPAST SILDeclRef.

When adding an AsyncFunctionPointer to the TBD, a LinkEntity of kind
AsyncFunctionPointerAST is constructed containing the decl.  That decl
is then wrapped in a SILDeclRef which is then mangled.

Previously, the kind of the SILDeclRef was erroneously forced to be
Func.  The result was a failure to mangle the TBD symbol for async
constructors correctly.

Here, that argument is omitted so that the kind can be determined
appropriately by SILDeclRef's constructor.

rdar://80485869

Author: nate-chandler

lib/IRGen/Linking.cpp

@@ -468,9 +468,7 @@ std::string LinkEntity::mangleAsString() const {
 
   case Kind::AsyncFunctionPointerAST: {
     std::string Result;
-    Result =
-        SILDeclRef(const_cast<ValueDecl *>(getDecl()), SILDeclRef::Kind::Func)
-            .mangle();
+    Result = SILDeclRef(const_cast<ValueDecl *>(getDecl())).mangle();
     Result.append("Tu");
     return Result;
   }

test/TBD/rdar80485869.swift

@@ -0,0 +1,7 @@
+// RUN: %empty-directory(%t)
+// RUN: %target-swift-frontend %s -emit-module-path %t/module -emit-tbd -enable-testing
+
+public actor Tom {
+    init() async {
+    }
+}