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- This way we can keep up the invariant that all entities are childs to
  a compound type

- Compound type selection is performed with Member
- Array indexing is performed with Sel
- Instead of the simpleSel constructor functions you can use the normal Member
  constructors now
- The concept of array element entities has been removed, Sel nodes just
  contain a pointer to the array type now.

Didn't notice this before because gcc doesn't report a warning for this,
clang does.

Apparently it is optional on linux (just often used because the RIP
encoding is often shorter than a global encoding).

This removes the strange entity/type duality and we get rid of union symconst_symbol.