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

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https://github.com/Sirraide commented:

Looks reasonable, but I’d prefer it if someone more familiar w/ the C standard than me could take another look at this.
CC @AaronBallman 

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