Refactor the module map file used for uniquing a module name out of

class Module. It's almost always going to be the same as
getContainingModule() for top-level modules, so just add a map to cover
the remaining cases.  This lets us do less bookkeeping to keep the
ModuleMap fields up to date.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@215268 91177308-0d34-0410-b5e6-96231b3b80d8