PersistentStateExpressions (e.g. ClangPersistentVariables) have the
ability to define types using expressions that persist throughout the
debugging session. GetCompilerTypeFromPersistentDecl is a useful
operation to have if you need to use any of those persistently declared types,
like in CommandObjectMemory.
This decouples clang from CommandObjectMemory and decouples Plugins from
Commands in general.