[llvm] [Support] Optimize signal handling file removal code (PR #173586)

[Fangrui Song via llvm-commits]

MaskRay wrote:

If we want to optimize `insert()`, we can introduce another intrusive link  `Node*          FreeNext;  // free list link. Touched ONLY under Lock` invisible to the signal handler, replacing the previous separate linked list. However, I don't think it's necessary since erase() is already O(N).

https://github.com/llvm/llvm-project/pull/173586