Ascent build system based on LLVM clang

[nkoukpaizan]

This PR adds the ascent-llvm CMake preset to build with LLVM clang on Ascent.

Closes #13

[nkoukpaizan]

There was a failing test on Ascent earlier. It passed when I re-ran, but that raises some flags for me.

[pelesh]

There was a failing test on Ascent earlier. It passed when I re-ran, but that raises some flags for me.

This example is nondeterministic due to algorithm used in cusolverRf library. We need to figure out how to set better pass/fail criteria for that test. Thanks for flagging it.

[pelesh]

There was a failing test on Ascent earlier. It passed when I re-ran, but that raises some flags for me.

This issue was fixed by @kswirydo in #109.

@nkoukpaizan, thanks for drawing attention to this.

[pelesh]

I've got a couple of minor suggestions to consider, but this looks good.

[pelesh]

Probably a good idea to rebase wrt develop since the bug that is tripping KLU+Rf+FGMRES test is fixed there. It is also okay to just squash and merge imho.

[cameronrutherford]

@ryandanehy or @nkoukpaizan or myself should be able to add a separate Ascent testing pipeline with this new compiler configuration. Might require some small re-arch, but should be doable. Can be addressed in another PR if not immediately doable.

[pelesh]

@ryandanehy or @nkoukpaizan or myself should be able to add a separate Ascent testing pipeline with this new compiler configuration. Might require some small re-arch, but should be doable. Can be addressed in another PR if not immediately doable.

No need for that b/c it is getting out of service soon.

[cameronrutherford]

LGTM - no need to test on Ascent since we are losing platform on Monday

[pelesh]

@nkoukpaizan, could you rebase to develop just to make sure all tests indeed pass. We can merge after that.