[clang] Exclude RedirectingFileSystem with null OverlayFileDir in VFSUsage (PR #128267)

[Hiroshi Yamauchi via cfe-commits]

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@@ -149,11 +149,16 @@ std::vector<bool> HeaderSearch::collectVFSUsageAndClear() const {
 
   llvm::vfs::FileSystem &RootFS = FileMgr.getVirtualFileSystem();
   // TODO: This only works if the `RedirectingFileSystem`s were all created by
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hjyamauchi wrote:

This PR partially addresses the limitation described by this comment.

https://github.com/llvm/llvm-project/pull/128267