[mlir][Linalg] Add tensor support to Linalg EDSC Builders

Summary:
This diff extends the Linalg EDSC builders so we can easily create mixed
tensor/buffer linalg.generic ops. This is expected to be useful for
HLO -> Linalg lowering.

The `StructuredIndexed` struct is made to derive from `ValueHandle` and can
now capture a type + indexing expressions. This is used to represent return
tensors.

Pointwise unary and binary builders are extended to allow both output buffers
and return tensors. This has implications on the number of region arguments.

Reviewers: ftynse, herhut, hanchung, asaadaldien, stellaraccident

Reviewed By: asaadaldien

Subscribers: merge_guards_bot, mehdi_amini, rriddle, jpienaar, burmako, shauheen, antiagainst, arpith-jacob, mgester, lucyrfox, aartbik, liufengdb, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D72863