[llvm] [BOLT][DWARF][NFC] Change how CloneUnitCtxMap is re-sized (PR #75876)

[Amir Ayupov via llvm-commits]

https://github.com/aaupov approved this pull request.

I'd suggest more explicit title e.g.: 
"[BOLT][DWARF][NFC] Initialize CloneUnitCtxMap with current partition size" 

https://github.com/llvm/llvm-project/pull/75876