[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.

Author: vsapsai

clang/lib/Lex/ModuleMap.cpp

@@ -310,8 +310,10 @@ 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) {
-    // Ignore missing excluded header files. They're optional anyway.
+  } 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 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

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

clang/test/Modules/missing-header.m

@@ -8,6 +8,9 @@
 @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'