llvm-4: small improvements on Darwin and elsewhere

Split outputs because there's no point in keeping a reference to Python
and it causes trouble during the Darwin stdenv bootstrap. There's also
an unnecessary dependency on LLVM in libc++ which causes us to rebuild
LLVM several more times than necessary during bootstrap, and an awkward
dependency on XPC in the TSAN that we turn off. This is in preparation
for using LLVM 4 in the Darwin stdenv and by default across nixpkgs.

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