Fix several ICEs related to malformed `#[repr(...)]` attributes
This PR fixes#83921. #83921 actually contains two related but distinct issues (one of them incorrectly reported as a duplicate in https://github.com/rust-lang/rust/issues/83921#issuecomment-814640734):
In the first, a call to `delay_span_bug` leads to an ICE when compiling with `-Zunpretty=everybody_loops` (and some other pretty-printing modes), because the corresponding error is emitted in a later pass, which does not run when only pretty-printing is requested.
The second issue is about parsing `#[repr(...)]` attributes. Currently, all of the following cause an ICE when applied to a struct/enum:
```rust
#[repr(packed())]
#[repr(align)]
#[repr(align(2, 4))]
#[repr(align())]
#[repr(i8())]
#[repr(u32(42))]
#[repr(i64 = 2)]
```
I have fixed this by expanding the well-formedness checks in `find_repr_attrs()`.
945458d472