Rollup merge of #145791 - samueltardieu:fix-zst-to-enum-mir-validation, r=compiler-errors

Fix ICE when validating transmuting ZST to inhabited enum

MIR validation attempts to determine the number of bytes needed to represent the size of the source type to compute the discriminant for the inhabited target enum. For a ZST source, there is no source data to use as a discriminant so no proper runtime check can be generated.

Since that should never be possible, insert a delayed bug to ensure the problem has been properly reported to the user by the type checker.

Fixes rust-lang/rust#145786

Author: samueltardieu