[lldb] Remove ClangASTImporter from Target

[lldb] Remove ClangASTImporter from Target

Target is one of the classes responsible for vending ClangASTImporter.
Target doesn't need to know anything about ClangASTImporter, so if we
instead have ClangPersistentVariables vend it, we can preserve
existing behavior while improving layering and removing dependencies
from non-plugins to plugins.