[llvm] [BOLT][DWARF][NFC] Fix DebugStrOffsetsWriter (PR #100672)

[Alexander Yermolovich via llvm-commits]

ayermolo wrote:

For the test, all we care about are strings (since that's what bug is about). Please reduce the test, and name it more appropriately. 

https://github.com/llvm/llvm-project/pull/100672