[IR] Require intrinsic struct return type to be anonymous

This is an alternative to D122376. Rather than working around the
problem, this patch requires that struct return types in intrinsics
are anonymous/literal and adds auto-upgrade code to convert
existing uses of intrinsics with named struct types.

This ensures that the mapping between intrinsic name and
intrinsic function type is actually bijective, as it is supposed
to be.

This also fixes https://github.com/llvm/llvm-project/issues/37891.

Differential Revision: https://reviews.llvm.org/D122471