[bazel] Fully replace gentbl with gentbl_cc_library (EnzymeAD#2231)

* [bazel] Fully replace gentbl with gentbl_cc_library

Enzyme uses tablegen generators from both llvm/tblgen.bzl and mlir/tblgen.bzl.
This consolidates usage so that we only use one of them (from MLIR).
Later we may delete the llvm/tblgen.bzl entirely.

This is mostly mechanical, but as a side effect, we need to adjust `includes`
to pick up `bundled_includes.h`. It was probably relying on an `includes`
config from the llvm/tblgen.bzl implementation that is no longer being provided.

See also llvm/llvm-project#124996.

* Write generated files to subdirectories (matching where cmake puts them) and use strip_include_prefix to mirror how they're referenced. Avoids needing to use `includes = ["."]` which is overly broad, and leads to a build failure.

Author: rupprecht