Merge "Build darwin_arm64 from source" am: 21204cd2c8 am: 7d0af1d166 am: 20e31dab32

Original change: https://android-review.googlesource.com/c/platform/external/llvm/+/1885368

Change-Id: I1cfa60cea6d2f81c18b1cf58eff8c9bff512ec20