[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:

Are your `textual` headers meant to be included more than once? If not, do they use header guards or `#pragma once`?

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