[llvm-debuginfo-analyzer] Use of std::unordered_set instead of std::set.

[CarlosAlbertoEnciso]

Extracted from llvm/tools/llvm-debuginfo-analyzer/README.txt
https://reviews.llvm.org/D125784#inline-1221421

Replace the std::set usage for DeducedScopes, UnresolvedScopes and
IdentifiedNamespaces with std::unordered_set and get the benefit
of the O(1) while inserting/searching, as the order is not important.

[llvmbot]

@llvm/issue-subscribers-debuginfo

Author: Carlos Alberto Enciso (CarlosAlbertoEnciso)

Extracted from [llvm/tools/llvm-debuginfo-analyzer/README.txt](https://github.com/llvm/llvm-project/blob/main/llvm/tools/llvm-debuginfo-analyzer/README.txt) https://reviews.llvm.org/D125784#inline-1221421

Replace the std::set usage for DeducedScopes, UnresolvedScopes and
IdentifiedNamespaces with std::unordered_set and get the benefit
of the O(1) while inserting/searching, as the order is not important.