[clang] [clang][modules] Headers meant to be included multiple times can be completely invisible in clang module builds (PR #83660)

[Ian Anderson via cfe-commits]

ian-twilightcoder wrote:

I don't really think it's the same thing. The problem I'm trying to fix is that nobody knows when it's appropriate to use `#import` vs `#include`, and the unfortunate convention of Objective-C makes it impossible for header owners to indicate if they support being included multiple times or not, as using header guards or `#pragma once` is very "un-Objective-C". Marking the header as `textual` is a fairly reasonable way for header owners to indicate that their header is meant to be included multiple times (e.g. stddef.h), even if ObjC developers don't know that and used `#import`. (The other use of `textual` that I've seen is for headers like `unwind_arm_ehabi.h` that are only allowed to have a single includer, and aren't really standalone.) If you don't want the header to build with the module, that's what `excluded` is for is it not?

And alternative solution would be @jansvoboda11's https://github.com/llvm/llvm-project/pull/71117, but I believe that will cause issues with non-modular headers building into multiple modules.

https://github.com/llvm/llvm-project/pull/83660