[mlir][Linalg] Extend generic ops to allow tensors

    Summary:
    This diff adds support to allow `linalg.generic` and
    `linalg.indexed_generic` to take tensor input and output
    arguments.

    The subset of output tensor operand types must appear
    verbatim in the result types after an arrow. The parser,
    printer and verifier are extended to accomodate this
    behavior.

    The Linalg operations now support variadic ranked tensor
    return values. This extension exhibited issues with the
    current handling of NativeCall in RewriterGen.cpp. As a
    consequence, an explicit cast to `SmallVector<Value, 4>`
    is added in the proper place to support the new behavior
    (better suggestions are welcome).

    Relevant cleanups and name uniformization are applied.

    Relevant invalid and roundtrip test are added.

    Reviewers: mehdi_amini, rriddle, jpienaar, antiagainst, ftynse

    Subscribers: burmako, shauheen, llvm-commits

    Tags: #llvm

    Differential Revision: https://reviews.llvm.org/D72022