Merge remote-tracking branch 'origin/master' into llvm11
Conflicts:
cmake/Findllvm.cmake
The llvm11 branch changed 10's to 11's and master branch added the
"using LLVM_CONFIG_EXE" help message, so the resolution was to merge
these changes together.
I also added a check to make sure LLVM is built with AVR enabled, which
is no longer an experimental target.