Add Enzyme AD support

[AVanDeBrook]

This PR adds a script to pull down specific releases of LLVM and Enzyme and build local versions of both.

On my machines LLVM compiles without issue, however, at run time it depends on gcc and/or g++ headers. I found that gcc-12 and g++-12 provide the most stability with llvm-16.

With the versions pinned in the shell script, Enzyme should compile without issue and the tests will output 2 errors; these errors do not affect the functionality of Enzyme.

Programs can be compiled like the following:

clang test.c -fplugin=.enzyme_build/Enzyme/enzyme/build/Enzyme/ClangEnzyme-16.so

Note(s):

• Enzyme has had a recent burst in development, so the most current version is ahead of the one I've tested and verified
• This uses LLVM version 16.0.0, Enzyme version 0.0.133, and depends on GCC/G++ version 12

[reid-g]

Running the enzyme_support.sh script starting from nothing only clones in the Enzyme repo. The llvm repo does not clone. Then crashes since there is no llvm repo.

[stonecoldhughes]

The two Dockerfiles below and associated shell scripts ilustrate 2 builds of Enzyme that pass all or most of the Enzyme builtin tests: "check-enzyme-integration" and "check-enzyme". One dockerfile passes one set of tests, and the other passes the other set of tests. The file extensions have all been converted to ".txt" so that Github will allow me to attach them in this comment. This is not in a PR because it is incomplete work; these illustrate the ENZYME tests. Next steps:

1. build a sample program and take the df/dx with ENZYME
2. build GridKit in the container; apply enzyme to gridkit as desired

resolve_enzyme_1.txt
resolve_enzyme_0.txt
run_1.txt
build_1.txt
build_0.txt
run_0.txt

container_1:

check_integration:

check enzyme:

container_0:

[stonecoldhughes]

This PR represents a good collection of Enzyme AD build/test examples using LLVM built from scratch or by package managers. I think it should be converted into an issue, since the scope is broad and exploratory, and what we want to do is still being investigated.
Some questions:

1. How can users get EnzymeAD? It is not a trivial installation process. Can a spack package handle it? Or a container environment? I have not attempted to build LLVM from scratch; with package managers, the installation is very sensitive to OS version. Is this also true when building from scratch, as in the build script?
2. How do we want to use Enzyme in the source code? Is there existing functionality that enzyme could replace, and a test to ensure that functionality remains operational?