Split Rank1 MdArray allocation into a separate helper (#84205)

Split Rank1 MdArray allocation into a separate helper (#84205)

MdArray rank1 with all-zero lower bounds gets actually allocated as an SzArray. On Native AOT this means we need to spin up to the type loader to potentially load the new type. Split this expensive (and impossible-to-express-in-C#) thing into a separate helper so we can trim the type loader.

https://github.com/dotnet/runtime/pull/84156#issuecomment-1491402863