[clang] [Modules] Don't fail when an unused textual header is missing. (PR #138227)

[Volodymyr Sapsai via cfe-commits]

vsapsai wrote:

Do you include proto headers directly or through the wrapper headers?

Are there any important macros in proto headers?

Do you need to abuse macros to get something out of proto headers? Kinda like `#define private public` but more realistic.

My initial reaction is to offer a solution by not using private headers (especially that it is so easily subverted by textual headers) and to rely on `-fmodules-embed-all-files` (which you already do). I believe you have constraints that make this "simple" solution not feasible. So I'm curious to know what about this solution doesn't work.

https://github.com/llvm/llvm-project/pull/138227