llvmPackages_12: build from monorepo source

As mentioned in #305146, keeping LLVM 12 is a source of pain because it
is the only version to be built from individual release tarball instead
of the LLVM monorepo. This commit makes LLVM 12 start from the monorepo
as well, simplifying all common LLVM expressions in the process.

With #347887, some quirks in the expressions for LLVM <14 were ironed
out, so building LLVM through from the monorepo is quite simple now.

- Most expressions only required minor changes, mostly removing the
  special casing for `sourceRoot`.

- The patch lists from llvm/12/default.nix were ported to
  common/default.nix. This only required a few extra conditionals which
  could be reduced via a rebuild also involving other LLVM versions.
  Outstanding tasks of little urgency have been noted in TODO comments.
  I have verified that the patch lists stay the same for all packages
  except LLVM where merely the order changes. An extra set of eyes
  is appreciated, of course.

- clang: The expression was reworked to use the same symlink location
  for clang-tools-extra for all versions including LLVM 12. This
  required adjusting the ad hoc patching of the clangd cmake files
  slightly.

- libunwind: We no longer need to make the libcxx sources available
  manually. We can rely on the monorepo source instead.

- lld: We no longer need to make the libunwind sources available manually.

- llvm: We no longer need to make the polly sources available manually

- On Darwin, we need to bypass CMake's C++ compiler for libcxx and
  libunwind now. It isn't a 100% clear why, probably because we've
  started to use Darwin's bootstrapStdenv for libcxx in the common
  expression compared to LLVM 12 on master [1].
  The reordering of flags for wasm causes a rebuild for some packages
  like firefox, but this should be tolerable on staging.

[1]: https://github.com/NixOS/nixpkgs/blob/665ebfb253caba7b85c2affefe2a92b305def4e6/pkgs/development/compilers/llvm/12/default.nix#L392-L430

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