clang/Modules: Rename CompilerInstance::ModuleManager, NFC

Fix the confusing naming of `CompilerInstance::ModuleManager`.  This is
actually an instance of `ASTReader`, which contains an instance of
`ModuleManager`.  I have to assume there was a point in the past where
they were just one class, but it's been pretty confusing for a while.  I
think it's time to fix it.

The new name is `TheASTReader`; the annoying `The` prefix is so that we
don't shadow the `ASTReader` class.  I tried out `ASTRdr` but that
seemed less clear, and this choice matches `ThePCHContainerOperations`
just a couple of declarations below.

Also rename `CompilerInstance::getModuleManager` and
`CompilerInstance::createModuleManager` to `*ASTReader`, making some
cases of `getModuleManager().getModuleManager()` a little more clear.

https://reviews.llvm.org/D70583