[8.0.0 Release] Release Candidate 3 source, docs, and binaries available

Hello everyone,

Source code, docs, and (some) binaries for LLVM-8.0.0-rc3 are now
available at https://prereleases.llvm.org/8.0.0/#rc3

More binaries will be added as they become available.

This is probably very close to what the final release will look like,
so please try it out, take a careful look at the docs etc., and report
any problems as bugs marked blocking https://llvm.org/pr40331