[Clang] Assignability of statement-expression struct result diverges from GCC

```c
int main() {
struct { int i; } s;
({ s; }).i = 7; // GCC allow, Clang reject
}
```
https://c.godbolt.org/z/W5zv7rMaY

Start by compiling the provided C reproducer with Clang and GCC and compare the diagnostics for assigning through the statement-expression result. The issue names no source file or test, so trace Clang's statement-expression and member-assignment handling; done means Clang accepts the assignment consistently with GCC and has regression coverage.

Written by the indexing model from the issue text.