Allow suppression of incompatible compiler warning

[dime10]

As discussed here, Enzyme can be compiled with non-LLVM compilers, as long as it is loaded/linked into LLVM tools (clang, opt, ...).

This PR adds a CMake option to suppress the compiler warning introduced in #1883.

[ZuseZ4]

cc @tgymnich, not sure why the Required Integration tests aren't running here, I approved the workflow.

[tgymnich]

@ZuseZ4 I guess this must have been a planned brownout of ubuntu 20.04 runners from GitHub. Here is the fix: #2262. PR is not merged because we still need to decide wether we want to keep C/C++ CI as required or only run C/C++ CI when a relevant file changes (see paths in PR). Both do not work together since the Required CI pass may never run if you don't touch any of the files define in the workflow.

Feel free to merge the PR without the paths: part.

[ZuseZ4]

@tgymnich I removed the path but CI there was blocked too, so I'm unable to merge it, sorry.
However it seems like the 20.04 runners here came back at some point.
So to not have this PR blocked too long and since no one seems to object I've added it to the merge queue, so they don't have to deal with extra warnings.