llvm is currenly built from individual tarballs instead of upstream monorepo

The flaws of this approach are:

1. there are boilerplate to reconstruct fragments of git monorepo in every subproject's .nix file. For example

   nixpkgs/pkgs/development/compilers/llvm/11/clang/default.nix

   Lines 14 to 20 in 9ddb9c0

   	unpackPhase = ''
   	unpackFile $src
   	mv clang-* clang
   	sourceRoot=$PWD/clang
   	unpackFile ${clang-tools-extra_src}
   	mv clang-tools-extra-* $sourceRoot/tools/extra
   	'';

2. there are difficulties with cherry-picking upstream patches because directory structure is altered, so nixpkgs has to host .patch-files instead of using fetchpatch from upstream git repo: https://github.com/NixOS/nixpkgs/tree/9ddb9c0021ede97e981c3dee2250bbd1abc50c6f/pkgs/development/compilers/llvm/11
3. it is not possible to use bleeding edge features which are already on git's master but not yet in the latest tarballed release. chromiumDev often needs them. With git repo, llvm_11.override{src = ...} would work.