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