[clang] [clang] Stub out gcc_struct attribute (PR #71148)

[Martin Storsjö via cfe-commits]

mstorsjo wrote:

> Microsoft bit-field layout didn't break an overly-specific regression test but rendered unusable double to string conversion. The culprit was the following snippet:
> 
> ```c++
> union Extractor {
>   double value;
>   struct {
>     bool sign : 1;
>     u32 exponent : 11;
>     u64 mantissa : 52;
>   };
> };
> ```
> 
> According to MSVC ABI, there should be padding between fields. I hope you agree that this is not an intuitive and expected behavior.

It is indeed an unexpected thing. However it is possible to work around it for all these cases; if you declare the inner structure like this:
```
struct {
  u64 sign : 1;
  u64 exponent : 11;
  u64 mantissa : 52;
};
```
Then there will be no extra padding between the fields.


https://github.com/llvm/llvm-project/pull/71148