[ROCm] Adding ROCDL Dialect.

This commit introduces the ROCDL Dialect (i.e. the ROCDL ops + the code to lower those ROCDL ops to LLWM intrinsics/functions). Think of ROCDL Dialect as analogous to the NVVM Dialect, but for AMD GPUs. This patch contains just the essentials needed to get a simple example up and running. We expect to make further additions to the ROCDL Dialect.

This is the first of 3 commits, the follow-up will be:
 * add a pass that lowers GPU Dialect to ROCDL Dialect
 * add a "mlir-rocm-runner" utility

Closes tensorflow/mlir#146

COPYBARA_INTEGRATE_REVIEW=https://github.com/tensorflow/mlir/pull/146 from deven-amd:deven-rocdl-dialect e78e8005c75a78912631116c78dc844fcc4b0de9
PiperOrigin-RevId: 271511259