Modules: Enforce that ModuleManager::removeModules deletes the tail

Modules: Enforce that ModuleManager::removeModules deletes the tail

ModuleManager::removeModules always deletes a tail of the
ModuleManager::Chain.  Change the API to enforce that so that we can
simplify the code inside.

There's no real functionality change, although there's a slight
performance hack to loop to the First deleted module instead of the
final module in the chain (skipping the about-to-be-deleted tail).

Also document something suspicious: we fail to clean deleted modules out
of ModuleFile::Imports.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@293398 91177308-0d34-0410-b5e6-96231b3b80d8