[lldb] Make CompleteTagDeclsScope completion order deterministic

Summary:
We iterate over `m_decls_to_complete` to complete declarations. As
`m_decls_to_complete` is a set the iteration order can be non-deterministic.
The order is currently only non-deterministic when we have
a large set of decls that need to be completed (i.e. more than 32 decls,
as otherwise the SmallPtrSet is just a linear-searched list).

This doesn't really fix any specific bug or has any really observable
change in behavior as the order in which we import should not influence
any semantics. However the order we create decls/types is now always
deterministic which should make debugging easier.

Reviewers: labath, mib, shafik, davide

Reviewed By: shafik, davide

Subscribers: davide, abidh, JDevlieghere, lldb-commits, mgrang

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D72495