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

This reverts commit 64bb60a.

Revert to give more time affected parties to adjust to the change.

Author: vsapsai

clang/lib/Lex/ModuleMap.cpp

@@ -310,10 +310,8 @@ void ModuleMap::resolveHeader(Module *Mod,
   } else if (Header.HasBuiltinHeader && !Header.Size && !Header.ModTime) {
     // There's a builtin header but no corresponding on-disk header. Assume
     // this was supposed to modularize the builtin header alone.
-  } else if ((Header.Kind == Module::HK_Excluded) ||
-             (Header.Kind == Module::HK_Textual)) {
-    // Ignore excluded and textual header files as a module can be built with
-    // such headers missing.
+  } else if (Header.Kind == Module::HK_Excluded) {
+    // Ignore missing excluded header files. They're optional anyway.
   } else {
     // If we find a module that has a missing header, we mark this module as
     // unavailable and store the header directive for displaying diagnostics.

clang/test/Modules/Inputs/submodules/module.modulemap

@@ -30,7 +30,3 @@ module missing_umbrella_with_inferred_submodules {
   module * { export * }
   export *
 }
-
-module missing_textual_header {
-  textual header "missing_textual.h"
-}

clang/test/Modules/missing-header.m

@@ -8,9 +8,6 @@
 @import missing_unavailable_headers.not_missing; // OK
 // CHECK-NOT: missing_unavailable_headers
 
-@import missing_textual_header; // OK
-// CHECK-NOT: missing_textual_header
-
 @import missing_headers;
 // CHECK: module.modulemap:15:27: error: header 'missing.h' not found
 // CHECK: could not build module 'missing_headers'