Enable new pass manager with LLVM 13
The new pass manager is enabled by default in clang since Clang/LLVM 13. Per the recent discussion on llvm-dev (https://lists.llvm.org/pipermail/llvm-dev/2021-August/152305.html) the legacy pass manager will be unmaintained in LLVM 14 and removed entirely in LLVM 15.
This switches us to use the new pass manager if LLVM >= 13 is used. It's possible to still use the old pass manager using `-Z new-llvm-pass-manager=no`.
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