Allow IndexType inside tensors.
It's common in many dialects to use tensors to themselves hold tensor shapes (for example, the shape is itself the result of some non-trivial calculation). Currently, such dialects have to use tensor<?xi64> or worse (like allowing either i32 or i64 tensors to represent shapes). tensor<?xindex> is the natural type to represent this, but is currently disallowed. This patch allows it.
Differential Revision: https://reviews.llvm.org/D76726