[lldb] Don't iterate over a std::set<Type*> in SymbolFileDWARF::GetTypes to make it deterministic

Summary:
Currently `SymbolFileDWARF::TypeSet` is a typedef to a `std::set<Type *>`.
In `SymbolFileDWARF::GetTypes` we iterate over a TypeSet variable when finding
types so that logic is non-deterministic as it depends on the actual pointer address values.

This patch changes the `TypeSet` to a `llvm::UniqueVector` which always iterates in
the order in which we inserted the types into the list.

Reviewers: JDevlieghere, aprantl

Reviewed By: JDevlieghere

Subscribers: mgrang, abidh, lldb-commits

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D75481