[PM] Move away from the old llvm/PassManager.h header and its using

declarations and just use the legacy namespace qualifier in this file
and the permanent header name.

The old wrapper header is going away to make LLVM's build more modular,
and without updating Clang I can't easily start to add usage of the new
pass manager to Clang. This should make it more clear in the code which
set of types is doing what.

llvm-svn: 229090