Materialize IndexType in the API.

Previously, index (aka affint) type was hidden under OtherType in the type API.
We will need to identify and operate on values of index types in the upcoming
MLFunc->CFGFunc(->LLVM) lowering passes.  Materialize index type into a
separate class and make it visible to LLVM RTTI hierarchy directly.
Practically, index is an integer type of unknown bit width and is accetable in
most places where regular integer types are.  This is purely an API change that
does not affect the IR.

After IndexType is separated out from OtherType, the remaining "other types"
are, in fact, TF-specific types only.  Further renaming may be of interest.

PiperOrigin-RevId: 220614026