Replace unordered_map with a vector. Pre-parse the section to statically
allocate storage. Use BumpPtrAllocator for FuncName strings, keep
StringRef in FuncDesc.
Reduces peak RSS of pseudo probe parsing from 9.08 GiB to 8.89 GiB as
part of perf2bolt with a large binary.
Test Plan:
```
bin/llvm-lit -sv test/tools/llvm-profgen
```
Reviewers: wlei-llvm, rafaelauler, dcci, maksfb, ayermolo
Reviewed By: wlei-llvm
Pull Request: https://github.com/llvm/llvm-project/pull/102905
a79cf0228e