Build with LLVM on Ascent

[pelesh]

A simple script to build ReSolve with LLVM/clang++ as the host compiler on Ascent

``` #!/bin/bash -e

module reset
module load cmake
module load llvm/12.0.0
module load cuda/11.4.2
module load suite-sparse/5.10.1

cmake
-DCMAKE_C_COMPILER=clang
-DCMAKE_CXX_COMPILER=clang++
-DCMAKE_INSTALL_PREFIX=../resolve_install
-DCMAKE_BUILD_TYPE=RelWithDebInfo
-DCMAKE_CUDA_ARCHITECTURES="70"
-DCMAKE_CUDA_COMPILER=nvcc
-DCMAKE_CUDA_HOST_COMPILER=clang++
-DCMAKE_CUDA_FLAGS=-allow-unsupported-compiler
../resolve

make VERBOSE=1 -j
make install
make test```

It is possible to modify the build system to build with LLVM/clang++ (instead of GCC). I started implementing this in the nicholson/ascent-llvm branch. See the main commit to see the changes. Moving forward, we have to think about how we want to organize the build system to support different platforms and stacks.

In the modified build system, the address sanitizer can be activated by setting RESOLVE_USE_ASAN=ON. See https://code.ornl.gov/ecpcitest/exasgd/resolve/-/blob/nicholson/ascent-llvm/buildsystem/ascent-llvm-cache.cmake#L29.

CC @nkoukpaizan

[pelesh]

@cameronrutherford says: We have a few approaches.
The easiest is to just add two things:

Build configuration to CMakePresets.json that contains these CMake args as inputs
Say you name your preset ascent-clang, then ascent-clang-env.sh would need to be created that contains your base modules

With this, MY_CLUSTER=ascent-clang ./buildsystem/build.sh will build and test out of the box.

Harder option is to also integrate with spack. Since there are system modules to leverage here, thankfully this isn't required. Would only be helpful for ensuring spack doesn't do anything weird with clang.