[clang] [Clang] Implement P3034R1 Module Declarations Shouldn’t be Macros (PR #90574)

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yronglin wrote:

> I am also seeing breakages which are solved by reverting this change.
> 
> The failure occurs when importing modules from within header files:
> 
> ```
> error: expected a module name after 'import'
>    11 | import std;
> ```

It would be nice to have a reproducer. I can't tell the root cause from this error, so I can't use it to strengthen the current tests.

https://github.com/llvm/llvm-project/pull/90574