[lldb] Move ClangModulesDeclVendor ownership to ClangPersistentVariables from Target

More decoupling of plugins and non-plugins. Target doesn't need to
manage ClangModulesDeclVendor and ClangPersistentVariables is always available
in situations where you need ClangModulesDeclVendor.

Differential Revision: https://reviews.llvm.org/D102811