[Classic Flang] LLVM 10-based Flang is ready to go

Hi list,

I am pleased to announce that the patches to make Classic Flang work with the LLVM 10 monorepo have been merged into https://github.com/flang-compiler/classic-flang-llvm-project (branch “release_100”). The build instructions at https://github.com/flang-compiler/flang/wiki/Building-Flang have been updated as well. Thanks very much to the help and reviews from Kiran, Gary, Peter, and Shivarama.