llvmPackages_15.llvm: run the tests on macOS

there are a few parts to this:
  - adding darwin specific check deps
  - working around referencing LLVM dylibs during the checkPhase in a
    way that supports darwin
    + previously we just set `$LD_LIBRARY_PATH` and/or made some
      strategic symlinks
    + now we have LLVM's `lit` config set the appropriate env vars as
      needed (as is done for other LLVM subprojects)
    + in retrospect switching to `installCheckPhase` might have been the
      better move..
  - patching `lit` to deal with `$DYLD_LIBRARY_PATH` being purged for
    new "protected" processes

more details within.

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