Fix clang crash and add bitfield support in __builtin_dump_struct.
In clang13.0.x, a struct with three or more members and a bitfield at the same time will cause a crash. In clang15.x, as long as the struct has one bitfield, it will cause a crash in clang.
Open issue: https://github.com/llvm/llvm-project/issues/54462
After this patch, this case no longer prints the field name. Eg: https://godbolt.org/z/o7vcbWaEf
now prints:
This seems like an important regression; please can you take a look? This also suggests to me that there's a hole in our test coverage.