[clang][CodeGen] Additional fixes for #114062 (#128166)
This addresses two issues introduced by moving indirect args into an
explicit AS (please see
<https://github.com/llvm/llvm-project/pull/114062#issuecomment-2659829790>
and
<https://github.com/llvm/llvm-project/pull/114062#issuecomment-2661158477>):
1. Unconditionally stripping casts from a pre-allocated return slot was
incorrect / insufficient (this is illustrated by the
`amdgcn_sret_ctor.cpp` test);
2. Putting compiler manufactured sret args in a non default AS can lead
to a C-cast (surprisingly) requiring an AS cast (this is illustrated by
the `sret_cast_with_nonzero_alloca_as.cpp test).
The way we handle (2), by subverting CK_BitCast emission iff a sret arg
is involved, is quite naff, but I couldn't think of any other way to use
a non default indirect AS and make this case work (hopefully this is a
failure of imagination on my part).
