[clang] [clang][deps] Fix dependency scanning with -working-directory (PR #84525)

[Ben Langmuir via cfe-commits]

benlangmuir wrote:

I don't think there's a difference we can test for here -- the VFS WD shouldn't be modified after the driver sets it and before the FM is used here, so it would be identical to `-working-directory` during the critical window.  But I agree recreating the FileManager is probably a better solution since it defines away this problem.  I'll look at doing it that way.

https://github.com/llvm/llvm-project/pull/84525