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

According to the documentation
> A header declaration that does not contain `exclude` nor `textual`
specifies a header that contributes to the enclosing module.

Which means that `exclude` and `textual` header don't contribute to the
enclosing module and their presence isn't required to build such a
module. The keywords tell clang how a header should be treated in a
context of the module but they don't add headers to the module.

When a textual header *is* used, clang still emits "file not found"
error pointing to the location where the missing file is included.

1 files changed, 4 insertions, 0 deletions

diff --git a/clang/test/Modules/Inputs/submodules/module.modulemap b/clang/test/Modules/Inputs/submodules/module.modulemap
index 1c1b76a08969..9e8143b8101d 100644
--- a/clang/test/Modules/Inputs/submodules/module.modulemap
+++ b/clang/test/Modules/Inputs/submodules/module.modulemap
@@ -30,3 +30,7 @@ module missing_umbrella_with_inferred_submodules {
   module * { export * }
   export *
 }
+
+module missing_textual_header {
+  textual header "missing_textual.h"
+}