Promote ModuleMap::Module to a namespace-scope class in the Basic

Promote ModuleMap::Module to a namespace-scope class in the Basic
library, since modules cut across all of the libraries. Rename
serialization::Module to serialization::ModuleFile to side-step the
annoying naming conflict. Prune a bunch of ModuleMap.h includes that
are no longer needed (most files only needed the Module type).

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@145538 91177308-0d34-0410-b5e6-96231b3b80d8