[Clang][Codegen] Do not annotate thunk's this/return types with align/deref/nonnull attrs

As it was discovered in post-commit feedback
for 0aa0458f,
we handle thunks incorrectly, and end up annotating
their this/return with attributes that are valid
for their callees, not for thunks themselves.

While it would be good to fix this properly,
and keep annotating them on thunks,
i've tried doing that in https://reviews.llvm.org/D100388
with little success, and the patch is stuck for a month now.

So for now, as a stopgap measure, subj.