[clang] Fix Wnested-anon-types in ABIArgInfo

D98794 added the DirectAttr/IndirectAttr struct fields to that union, but
declaring anonymous structs in an anonymous union triggers `-Wnested-anon-types`
warnings. We can't just give them a name as they are in an anonymous union, so
this just declares the type outside.

```
clang/include/clang/CodeGen/CGFunctionInfo.h:97:5: warning: anonymous types declared in an anonymous union are an extension [-Wnested-anon-types]
    struct {
    ^
clang/include/clang/CodeGen/CGFunctionInfo.h:101:5: warning: anonymous types declared in an anonymous union are an extension [-Wnested-anon-types]
    struct {
    ^
```

Reviewed By: chill

Differential Revision: https://reviews.llvm.org/D102903