[Clang] Bitfield result from statement-expression not integer-promoted, diverges from GCC

```c
#include

int main(void) {
struct { unsigned b : 3; } s;
printf("%d\n", _Generic(+({s.b;}), int:1, unsigned: 2)); // GCC 1, Clang 2
}
```

https://c.godbolt.org/z/6G7heY9W5

Start by reproducing the minimized C example in Clang and GCC, then investigate Clang's handling of bitfields, statement-expressions, and integer promotions. The issue is complete when the expression's _Generic result matches GCC's int classification while preserving the relevant language behavior.

Written by the indexing model from the issue text.