Move the old pass manager infrastructure into a legacy namespace and
give the files a legacy prefix in the right directory. Use forwarding
headers in the old locations to paper over the name change for most
clients during the transitional period.

No functionality changed here! This is just clearing some space to
reduce renaming churn later on with a new system.

Even when the new stuff starts to go in, it is going to be hidden behind
a flag and off-by-default as it is still WIP and under development.

This patch is specifically designed so that very little out-of-tree code
has to change. I'm going to work as hard as I can to keep that the case.
Only direct forward declarations of the PassManager class are impacted
by this change.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@194324 91177308-0d34-0410-b5e6-96231b3b80d8