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

[Volodymyr Sapsai via cfe-commits]

vsapsai wrote:

What about the approach to provide all_textual.pcm to use.cpp and for clang to find foo.h in all_textual.pcm? It doesn't work right now but I'm not sure that it shouldn't (there can be good reasons why it shouldn't work, tbh).

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