This patch to the Go frontend delays building the struct type for a
go/defer thunk until it is needed.  We were building it in the
determine_types pass.  Now, we delay until the
simplify_thunk_statements pass.  That ensures that we are consistent
in determining whether an argument is constant.

With this change we no longer need to add a field for a call to
recover, as the simplify_thunk_statements pass runs after the
build_recover_thunks pass, so the additional argument will already
have been added to the call.

The test case for this is https://go.dev/cl/440297.

This fixes https://go.dev/issue/56109.

Bootstrapped and ran Go testsuite on x86_64-pc-linux-gnu.  Committed
to mainline.

Ian