c++/modules: fix -MG for header units [PR123622]

With -MG we should allow a nonexistent header unit, as we do with a
nonexistent #include.  But still import it if available.

	PR c++/123622

gcc/cp/ChangeLog:

	* module.cc (preprocess_module): Check deps.missing_files.

gcc/testsuite/ChangeLog:

	* g++.dg/modules/dep-6.C: New test.

Co-authored-by: <mtxn@duck.com>

gcc/cp/module.cc

@@ -23319,7 +23319,13 @@ preprocess_module (module_state *module, location_t from_loc,
 	      if (!(import->is_module ()
 		    && (import->is_partition () || import->is_exported ()))
 		  && import->loadedness == ML_NONE
-		  && (import->is_header () || !flag_preprocess_only))
+		  && (!flag_preprocess_only
+		      || (import->is_header ()
+			  /* Allow a missing/unimportable GCM with -MG.
+			     FIXME We should also try falling back to #include
+			     before giving up entirely.  */
+			  && (!cpp_get_options (reader)->deps.missing_files
+			      || import->check_importable (reader)))))
 		{
 		  unsigned n = dump.push (import);
 		  import->do_import (reader, true);

gcc/testsuite/g++.dg/modules/dep-6.C

@@ -0,0 +1,6 @@
+// PR c++/123622
+// { dg-additional-options "-fmodules -M -MG -MF dep-6.d" }
+
+import "dep-6.h"; // Must not exist!
+
+// { dg-final { scan-file dep-6.d {\nCXX_IMPORTS \+= \./dep-6\.h\.c\+\+-module} } }