[Dependency Scanning] Remove hard failure mode (crash) during cross-import resolution

Author: artemcm

lib/DependencyScan/ModuleDependencyScanner.cpp

@@ -602,7 +602,7 @@ static void discoverCrossImportOverlayFiles(
   auto getModuleIDForImportIdentifier =
       [](const std::string &importIdentifierStr,
          const ModuleDependencyIDSet &directSwiftDepsSet,
-         const ModuleDependencyIDSet &directClangDepsSet) -> ModuleDependencyID {
+         const ModuleDependencyIDSet &directClangDepsSet) -> std::optional<ModuleDependencyID> {
     if (auto textualDepIt = directSwiftDepsSet.find(
             {importIdentifierStr, ModuleDependencyKind::SwiftInterface});
         textualDepIt != directSwiftDepsSet.end())
@@ -615,8 +615,8 @@ static void discoverCrossImportOverlayFiles(
                  {importIdentifierStr, ModuleDependencyKind::Clang});
              clangDepIt != directClangDepsSet.end())
       return *clangDepIt;
-    llvm_unreachable(
-        "Unresolved import during cross-import overlay resolution");
+    else
+      return std::nullopt;
   };
 
   // Collect the set of directly-imported module dependencies
@@ -625,9 +625,10 @@ static void discoverCrossImportOverlayFiles(
     auto importResolvedModuleID = getModuleIDForImportIdentifier(
         import.importIdentifier, mainModuleDirectSwiftDepsSet,
         mainModuleDirectClangDepsSet);
-    for (const auto &importLocation : import.importLocations)
-      perSourceFileDependencies[importLocation.bufferIdentifier].insert(
-          importResolvedModuleID);
+    if (importResolvedModuleID)
+      for (const auto &importLocation : import.importLocations)
+        perSourceFileDependencies[importLocation.bufferIdentifier].insert(
+          *importResolvedModuleID);
   }
 
   // For each source-file, build a set of module dependencies of the
@@ -658,9 +659,10 @@ static void discoverCrossImportOverlayFiles(
               auto importResolvedDepID = getModuleIDForImportIdentifier(
                   import.importIdentifier, directSwiftDepsSet,
                   directClangDepsSet);
-              if (!perSourceFileDependencies[bufferIdentifier].count(
-                      importResolvedDepID))
-                worklist.push_back(importResolvedDepID);
+              if (importResolvedDepID &&
+                  !perSourceFileDependencies[bufferIdentifier].count(
+                      *importResolvedDepID))
+                worklist.push_back(*importResolvedDepID);
             }
           }
         }

test/ScanDependencies/test_clang_gmodules.swift

@@ -1,6 +1,6 @@
 // RUN: %empty-directory(%t)
 // RUN: %empty-directory(%t/module-cache)
-// RUN: %target-swift-frontend -scan-dependencies -module-cache-path %t/module-cache %s -o %t/deps.json -I %S/Inputs/CHeaders -I %S/Inputs/Swift -target %target-cpu-apple-macosx10.14
+// RUN: %target-swift-frontend -scan-dependencies -module-cache-path %t/module-cache %s -o %t/deps.json -I %S/Inputs/CHeaders -I %S/Inputs/Swift -target %target-cpu-apple-macosx10.14 -disable-implicit-concurrency-module-import -disable-implicit-string-processing-module-import
 // RUN: %validate-json %t/deps.json | %FileCheck %s
 
 import X