[llvm-branch-commits] [hmaptool] Implement simple string deduplication (PR #102677)

[Shoaib Meenai via llvm-branch-commits]

https://github.com/smeenai created https://github.com/llvm/llvm-project/pull/102677

This reduces the size of the generated header maps significantly (35%
measured internally). Further savings are possible through tail
deduplication, but the additional complication isn't worth the gain IMO.