[clang] [clang] Only set non-empty bypass to scan VFS (PR #159605)

[Jan Svoboda via cfe-commits]

https://github.com/jansvoboda11 created https://github.com/llvm/llvm-project/pull/159605

Normalizing an empty modules cache path results in an incorrect non-empty path (the working directory). This PR conditionalizes more code to avoid this. Tested downstream by swift/llvm-project and the `DependencyScanningCAPITests.DependencyScanningFSCacheOutOfDate` unit test.

>From c90297a181f30d69caf1d701cab716e6be8b6875 Mon Sep 17 00:00:00 2001
From: Jan Svoboda <jan_svoboda at apple.com>
Date: Thu, 18 Sep 2025 10:05:03 -0700
Subject: [PATCH] [clang] Only set non-empty bypass to scan VFS

Normalizing an empty modules cache path results in an incorrect non-empty path (the working directory). This conditionalizes more code to avoid this. Tested downstream by swift/llvm-project and the `DependencyScanningCAPITests.DependencyScanningFSCacheOutOfDate` unit test.
---
 .../DependencyScanning/DependencyScanningWorker.cpp   | 11 ++++++-----
 1 file changed, 6 insertions(+), 5 deletions(-)

diff --git a/clang/lib/Tooling/DependencyScanning/DependencyScanningWorker.cpp b/clang/lib/Tooling/DependencyScanning/DependencyScanningWorker.cpp
index 0a12c479bf8e3..16cb56fddc006 100644
--- a/clang/lib/Tooling/DependencyScanning/DependencyScanningWorker.cpp
+++ b/clang/lib/Tooling/DependencyScanning/DependencyScanningWorker.cpp
@@ -445,13 +445,14 @@ class DependencyScanningAction {
 
     // Use the dependency scanning optimized file system if requested to do so.
     if (DepFS) {
-      SmallString<256> ModulesCachePath;
-      normalizeModuleCachePath(
-          *FileMgr, ScanInstance.getHeaderSearchOpts().ModuleCachePath,
-          ModulesCachePath);
       DepFS->resetBypassedPathPrefix();
-      if (!ModulesCachePath.empty())
+      if (!ScanInstance.getHeaderSearchOpts().ModuleCachePath.empty()) {
+        SmallString<256> ModulesCachePath;
+        normalizeModuleCachePath(
+            *FileMgr, ScanInstance.getHeaderSearchOpts().ModuleCachePath,
+            ModulesCachePath);
         DepFS->setBypassedPathPrefix(ModulesCachePath);
+      }
 
       ScanInstance.setDependencyDirectivesGetter(
           std::make_unique<ScanningDependencyDirectivesGetter>(*FileMgr));