Auto merge of #50317 - varkor:repr-align-assign, r=nagisa

Improve error message for #[repr(align=x)]

Before:
```
error[E0552]: unrecognized representation hint
 --> src/main.rs:1:8
  |
1 | #[repr(align="8")]
  |        ^^^^^^^^^
```
After:
```
error[E0693]: incorrect `repr(align)` attribute format
 --> src/main.rs:1:8
  |
2 | #[repr(align="8")]
  |        ^^^^^^^^^ help: use parentheses instead: `align(8)`
```

Fixes #50314.