Fix build using setVirtualFileSystem

[wenju-he]

Update per llvm commit
llvm/llvm-project@30633f308941

[Copilot]

Pull Request Overview

Updates the code to use the new LLVM API for setting a virtual file system, replacing the deprecated createFileManager(fs) method with separate calls to setVirtualFileSystem() and createFileManager().

• Replace deprecated createFileManager(fs) with new LLVM API pattern
• Split file system setup into two separate method calls as required by the updated LLVM interface

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