[clang] [llvm] [llvm][vfs] Allow root directory remap. Fix assertion failure. (PR #83766)

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yelleyee wrote:

Refinement: The current implementation suggests that the top `remap-entry` node must be a directory in the tree, which is not consistent with the rest of the tree design.    
In the reset nodes of the tree, both `directory` and `directory-remap` should be allowed. Therefore, the `uniqueOverlayTree` function should allow the `SrcE` parameter to be passed with a `EK_DirectoryRemap` kind parameter when initially entering the function.

https://github.com/llvm/llvm-project/pull/83766