[NFC] Derive from PassInfoMixin for no-op/printing passes

PassInfoMixin should be used for all NPM passes, rater than a custom
`name()`.

This caused ambiguous references in LegacyPassManager.cpp, so had to
remove "using namespace llvm::legacy" and move some things around.

The passes had to be moved to the llvm namespace, or else they would get
printed as "(anonymous namespace)::FooPass".

Reviewed By: ychen, asbirlea

Differential Revision: https://reviews.llvm.org/D83498