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
8039d2c3bf