Custom LLVM build

[rdong8]

Dreading this because the right way to do it is by creating a Conan package for LLVM instead of just shunting a CMake build into another docker stage. There is this guy but it doesn't look like it includes MLIR, so is not suitable because #14.

The reason why we want this is because the LLVM currently used is the one installed via dnf in the devcontainer, but that's built against libstdc++. The the linuxbrew package for LLVM does include MLIR but is also built against libstdc++ so it doesn't work either.

[rdong8]

Looking closer, we probably need to do a bootstrap build so that it links against its own libc++. LLVM already has some helpers to do custom builds in docker so that might be the path of least resistance

[rdong8]

https://stackoverflow.com/questions/79753528/weird-interaction-between-libstdc-standard-library-headers-and-module

Might also be solved by switching to libc++