[llvm] [LLVM][PDB] Use IsUnsigned flag for APInt correctly (PR #131598)

[Aleksandr Korepanov via llvm-commits]

AlexK0 wrote:

> A test case should be included - if it's reachable via pdb utilities in LLVM, that'd be ideal. If not, then maybe a unit test in LLVM using the API would be do-able? @zmodem might have pointers for a suitable reviewer for PDB related things

I didn't find a way to write a test using `llvm-pdbutil` (it seems it doesn't use this API). Therefore, I added simple C++ unittests.

https://github.com/llvm/llvm-project/pull/131598