add clang compiler support

[shurale-nkn]

add clang++ compiler support in Cmake file.
Clang++ as default compiler at CI.
required for full integration in MIOpen ROCm/MIOpen#1692

[illsilin]

There are 3 issues with this PR:

1. The compiler path is hardcoded to /opt/rocm/bin, which is going to use the rocm 5.1.0 release version. Meanwhile, our default compiler version is the "ck-9110" branch, and in the near future we may switch to the "amd-stg-open" branch.
2. The Client app is still built with hipcc.
3. The assembly code compiled with clang and hipcc appears to be different.

We have worked with Kamil to resolve the first two issues in the lwpck-379 branch, so we can create a PR similar to this one, that would allow us to build CK with the correct version of clang.
I have verified that the performance of the gemm and resnet50 tests is the same, or even slightly improved, as with the hipcc-compiled code when the correct compiler version is used.
However, we probably need to determine whether the differences in the assembly code are acceptable.

[shurale-nkn]

Yes, this PR will be replaced by a new one, based on branch lwpck-379.
As soon as a new PR is created, I will insert a link so as not to violate causal relationships.

[shurale-nkn]

replaced by #387