[clang] Add #pragma clang __debug module_lookup

clang/include/clang/Basic/DiagnosticLexKinds.td

@@ -715,6 +715,8 @@ def warn_pragma_debug_unexpected_command : Warning<
   "unexpected debug command '%0'">, InGroup<IgnoredPragmas>;
 def warn_pragma_debug_unknown_module : Warning<
   "unknown module '%0'">, InGroup<IgnoredPragmas>;
+def warn_pragma_debug_unable_to_find_module : Warning<
+  "unable to find module '%0'">, InGroup<IgnoredPragmas>;
 // #pragma module
 def err_pp_expected_module_name : Error<
   "expected %select{identifier after '.' in |}0module name">;

clang/lib/Lex/Pragma.cpp

@@ -1119,11 +1119,27 @@ struct PragmaDebugHandler : public PragmaHandler {
         M = MM.lookupModuleQualified(IIAndLoc.first->getName(), M);
         if (!M) {
           PP.Diag(IIAndLoc.second, diag::warn_pragma_debug_unknown_module)
-              << IIAndLoc.first;
+              << IIAndLoc.first->getName();
           return;
         }
       }
       M->dump();
+    } else if (II->isStr("module_lookup")) {
+      Token MName;
+      PP.LexUnexpandedToken(MName);
+      auto *MNameII = MName.getIdentifierInfo();
+      if (!MNameII) {
+        PP.Diag(MName, diag::warn_pragma_debug_missing_argument)
+            << II->getName();
+        return;
+      }
+      Module *M = PP.getHeaderSearchInfo().lookupModule(MNameII->getName());
+      if (!M) {
+        PP.Diag(MName, diag::warn_pragma_debug_unable_to_find_module)
+            << MNameII->getName();
+        return;
+      }
+      M->dump();
     } else if (II->isStr("overflow_stack")) {
       if (!PP.getPreprocessorOpts().DisablePragmaDebugCrash)
         DebugOverflowStack();

clang/test/Modules/clang-pragmas.c

@@ -0,0 +1,22 @@
+// RUN: rm -rf %t
+// RUN: split-file %s %t
+// RUN: %clang_cc1 -fmodules -fimplicit-module-maps -I%t %t/tu.c -fsyntax-only \
+// RUN:   -verify 2>&1 | FileCheck %s
+
+//--- module.modulemap
+
+module A {
+  header "A.h"
+}
+
+//--- A.h
+
+//--- tu.c
+
+#pragma clang __debug module_map A // expected-warning{{unknown module 'A'}}
+#pragma clang __debug module_lookup B // expected-warning{{unable to find module 'B'}}
+#pragma clang __debug module_lookup A // does header search for A
+#pragma clang __debug module_map A // now finds module A
+
+// CHECK: module A
+// CHECK: module A