[lld] [llvm] [FoldingSet] Add typed lookup/insert/erase/getOrInsert (PR #219644)

[Fangrui Song via llvm-commits]

MaskRay wrote:

This is stage 1 of a staged migration replacing `void *InsertPos` across FoldingSet's
callers with the typed `FoldingSetInsertToken`. Plan:

1. (this PR) Add lookup/insert/erase/getOrInsert alongside the existing `void *`-based names; the old names forward to the new ones.
2. Per-area follow-ups migrate callers one area at a time (SCEV, TableGen, clang AST/Sema, clang StaticAnalyzer, SelectionDAG, misc), each an independent NFC PR.
3. Once nothing calls the void * names, delete them.

@fhahn: #217571 also touches UniqueSCEVs's lookup site; happy to rebase around whichever lands first.

https://github.com/llvm/llvm-project/pull/219644