[clang] [Clang] Remove `IDNS_Ordinary` flag in `IndirectFieldDecl::IdentifierNamespace` (PR #100525)

[Aaron Ballman via cfe-commits]

https://github.com/AaronBallman approved this pull request.

The C standard does not make this particularly clear, but the changes are correct.

Per C23 6.2.1p4 Each member of the structure has file scope (because why not) but per 6.2.3p1, each member is in a distinct name space and the member is disambiguated by the type of the expression used to access the member via the `.` or `->` operator. That last bit is why the member cannot be an ordinary identifier within the structure itself.

So LGTM, thank you!

https://github.com/llvm/llvm-project/pull/100525