Fix passing self by value for types passed by value

For types that are passed by value, we can't just cast the value to a
pointer, but have to use an alloca and copy the value there. This
handling is already present for all other arguments, but was missing
for "self".

Fixes #6682, #4850 and #4878
auto merge of #6903 : dotdash/rust/self_by_value, r=catamorphism

For types that are passed by value, we can't just cast the value to a
pointer, but have to use an alloca and copy the value there. This
handling is already present for all other arguments, but was missing
for "self".

Fixes #6682 #4850 #4878