[Dependency Scanning] Avoid querying Swift Overlay for underlying module

Avoid Swift Overlay lookup for the underlying clang module of a known Swift module.
i.e. When computing set of Swift Overlay dependencies for module 'A', which depends on a Clang module 'A', ensure we don't lookup Swift module 'A' itself here - this can lead to bizarre interactions where the source module under scan is queried as a dependency of itself.

Resolves rdar://159706486

Author: artemcm

lib/DependencyScan/ModuleDependencyScanner.cpp

@@ -1456,7 +1456,7 @@ void ModuleDependencyScanner::resolveSwiftOverlayDependenciesForModule(
 
   // A scanning task to query a Swift module by-name. If the module already
   // exists in the cache, do nothing and return.
-  auto scanForSwiftDependency = [this, &cache, &lookupResultLock,
+  auto scanForSwiftDependency = [this, &moduleID, &cache, &lookupResultLock,
                                  &swiftOverlayLookupResult](
                                     Identifier moduleIdentifier) {
     auto moduleName = moduleIdentifier.str();
@@ -1467,6 +1467,13 @@ void ModuleDependencyScanner::resolveSwiftOverlayDependenciesForModule(
         return;
     }
 
+    // Avoid Swift overlay lookup for the underlying clang module of a known
+    // Swift module. i.e. When computing set of Swift Overlay dependencies
+    // for module 'A', which depends on a Clang module 'A', ensure we don't
+    // lookup Swift module 'A' itself here.
+    if (moduleName == moduleID.ModuleName)
+      return;
+
     auto moduleDependencies = withDependencyScanningWorker(
         [moduleIdentifier](ModuleDependencyScanningWorker *ScanningWorker) {
           return ScanningWorker->scanFilesystemForSwiftModuleDependency(

test/ScanDependencies/embedded_no_textual.swift

@@ -1,3 +1,4 @@
+// REQUIRES: swift_feature_Embedded
 // RUN: %empty-directory(%t)
 // RUN: split-file %s %t