[clang] Only set non-empty bypass to scan VFS (llvm#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.

Author: jansvoboda11

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));