Remove ExplicitCompletionHandlerIndex flag

Assuming that we don't typecheck a deserialized module, we don't
actually need the ExplicitCompletionHandlerIndex. We used this flag to
determine whether we could trust the number stored in the handler index
or just take the last parameter of the function we're attached to.

Since we only typecheck it before serialization, we can safely check
that the completion handler location is valid before choosing whether we
should trust it or not.

lib/Serialization/ModuleFormat.h

@@ -1926,7 +1926,6 @@ namespace decls_block {
 
   using CompletionHandlerAsyncDeclAttrLayout = BCRecordLayout<
     CompletionHandlerAsync_DECL_ATTR,
-    BCFixed<1>,                 // True if explicit handler index
     BCVBR<5>,                   // Completion handler index
     DeclIDField                 // Mapped async function decl
   >;