Fix createdump crash on alpine 3.12 and above. (#56272)

Fix createdump crash on alpine 3.12 and above. (#56272)

Finally hit an existing problem in createdump (since 2.1) where the std::set<MemoryRegions> in
ThreadInfo::GetThreadStack() where it is calling CrashInfo::SearchMemoryRegions with m_crashInfo.OtherMappings()
which returns a copy of the set of MemoryRegions instead of a reference. On alpine 3.12 musl, this set copy
is freed right away when it goes out of scope as soon as SearchMemoryRegions returns. On any other Linux distro
and MacOS the set doesn't get freed/invalidated as soon.

Return references to the threads and memory region sets in the CrashInfo functions.