llvmPackages_10: rc2 -> rc3

http://lists.llvm.org/pipermail/llvm-dev/2020-March/139729.html

Additionally cherry-picked 3 commits from `llvm-project/master`:
- llvm/llvm-project@d21664c
- llvm/llvm-project@3a0f6e6
- llvm/llvm-project@87dac7d

such that clang can automatically pick up the polly plugin from the
`llvm-polly` build.