[ScanDependency] Fix a regression caused by rewrite in swiftlang#76700

In the refactoring change swiftlang#76700, it accidentally introduced a behavior
change that causes the generated PCM command-line to have useful
VFSOverlay files getting dropped. Clang module command-line and its
unused VFS pruning should be done by the clang dependency scanner
already so there is no need to touch that in the swift scanner. Since
the original logics is not used to handle clang module commands, it will
actually dropped the useful vfs overlay that is needed when none of the
dependencies uses it. Fix the regression by restoring the old behavior
and ignoring clang modules when pruning VFS overlay.

rdar://139233781
(cherry picked from commit e5d0c207f19cb7be40019a74b8e5b02075410689)

Author: cachemeifyoucan

lib/DependencyScan/ScanDependencies.cpp

@@ -451,6 +451,11 @@ class ExplicitModuleDependencyResolver {
   }
 
   void pruneUnusedVFSOverlay() {
+    // Pruning of unused VFS overlay options for Clang dependencies is performed
+    // by the Clang dependency scanner.
+    if (moduleID.Kind == ModuleDependencyKind::Clang)
+      return;
+
     std::vector<std::string> resolvedCommandLine;
     size_t skip = 0;
     for (auto it = commandline.begin(), end = commandline.end();

test/ScanDependencies/preserve_used_vfs.swift

@@ -65,3 +65,10 @@ import F
 
 /// --------Clang module F
 // CHECK-LABEL: "modulePath": "{{.*}}{{/|\\}}F-{{.*}}.pcm",
+// CHECK: "commandLine": [
+// CHECK: "-vfsoverlay",
+// CHECK-NEXT: "{{.*}}{{/|\\}}preserve_used_vfs.swift.tmp{{/|\\}}overlay.yaml",
+// CHECK: "-ivfsoverlay",
+// CHECK-NEXT: "-Xcc",
+// CHECK-NEXT: "{{.*}}{{/|\\}}preserve_used_vfs.swift.tmp{{/|\\}}overlay.yaml",
+// CHECK: ],