Fix ICE in `offset_of!` error recovery Fixes rust-lang/rust#153236. `offset_of!` was changed in rust-lang/rust#148151 to lower through THIR as a sum of calls to the `offset_of` intrinsic. In the error-recovery case, when no valid field indices are recorded, that lowering synthesized `0` as a `u32` even though the overall `offset_of!` expression has type `usize`. On 64-bit targets, const-eval then tried to write a 4-byte immediate into an 8-byte destination, which caused the ICE.