[lldb][NativePDB] Remove cantFail uses (1 out of ?) (#187158)
This is a follow-up to
https://github.com/swiftlang/llvm-project/pull/12317#discussion_r2850297229
Per that discussion, given that deserializers *can* fail given a corrupt
PDB, it's preferable to handle the error instead of crashing.
This specific change is limited to "easy" changes (read: I have high
confidence in their correctness). The ideal end state is funneling all
errors to a few central places in `SymbolFileNativePDB`.