COMGR: turn on unit testing

[kasaurov]

Motivation

Add a non-blocking build step, which runs COMGR unit tests

Technical Details

• COMGR unit tests is set of small and fast GPU-agnostic (compilation only) compilation tests. They include LIT tests, which depends on source tree, so to run them on a separate runner will require copying lots of data. All these make they suitable to be run on the builder machine.
• the PR also includes a patch for libhipcxx which is not yet included into TheRock submodule
• $${\color{red}\text{EXPERIMENTAL}}$$: sets -DTHEROCK_ENABLE_LLVM_TESTS=ON for compiler-runtime

Test Plan

TBD

Test Result

TBD

Submission Checklist

• Look over the contributing guidelines at https://github.com/ROCm/ROCm/blob/develop/CONTRIBUTING.md#pull-requests.