[clang] [Clang] prevent incorrect rejection of auto with reordered declaration specifiers in C23 (PR #177865)

[Corentin Jabot via cfe-commits]

https://github.com/cor3ntin commented:

I am not sure this is the right approach.

- There are many more qualifiers (`_Atomic` for example, but look at `isTypeSpecifierQualifier` for more of the complexity)

I would like @AaronBallman to help us clarify the C23 rules. In particular as @Fznamznon mention, it is odd that `long auto long` would be allowed, although GCC accepts it. Is that a defect?

The tests should probably check if the behavior is consistent with static/extern

https://github.com/llvm/llvm-project/pull/177865