[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:

Wow that's a _ton_ of PCM files. Why are your headers `textual` if they're only meant to be included once? Do they need to inherit the includer's preprocessor environment or something like that?

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