meson: Treat LLVM headers as a system dependency
This will suppress warnings from LLVM includes, which we as a library consumer don't usually care about.
What motivates this change is trying to build RADV with -Wshadow to avoid future bugs (see !7430 (merged)). Silencing LLVM warnings enables us to do so, since currently the gcc version used for CI (8.3.0) triggers that diagnostic for some LLVM classes.