This method allows to create artificial layouts that are not directly
associated to `llvm::Value`s.
Because of this missing association with `Value`s, the artificial
layouts are not retrievable by using the public `getOrCreateLayoutType`
and `getLayoutType` methods of `LayoutTypeSystem`, but they will be
accessible only by walking the graph.
The capability to add layouts that are not associated to `llvm::Value`s
is necessary for the future implementation of the DLA Step:
`ComputeNonInterferingComponents`.
The step will add artificial layouts to represent non-intefering
components.
Pietro Fezzardi