gn build: Update TODO now that libcxx libcxxabi libunwind clang-tools-extra are done

[phosek]

LGTM although we're still missing tests for libunwind, libcxxabi and libcxx.

[thakis]

Thanks!

Are you going to work on those tests?

llvm/utils/gn/TODO.txt

 Ideas for things to do:
 
-- more projects (compiler-rt libcxx libcxxabi libunwind clang-tools-extra lldb)
-  - phosek expressed interest in compiler-rt libcxx libcxxabi libunwind
+- more projects (compiler-rt's asan/ubsan/msan/tsan, lldb)
   - once there are more projects, have an llvm_enable_projects arg, modeled
     after llvm_targets_to_build in the GN build
   - a check-all build target that runs test of all projects
 - more targets (AVR MIPS RISCV SystemZ etc)
   - example: https://reviews.llvm.org/D56416
 - investigate feasibility of working `gn check`
 
 - "optimized tblgen" mode
   - either just always build tablegen and support with opt config
   - or use opt toolchain and build tablegen twice in debug builds, like cmake
 
 - cross builds using GN's toolchain feature
-- one-build-dir bootstrap builds using GN's toolchain feature
 
 - move clang_tablegen into lib/ for private files
 - add dead code stripping
 - move run_tablegen.py from build to tablegen folder
 - figure out why -Iclang/Support gets added so often
 - plugin() template with working rpath, exports thingers
   - then port clang_build_examples and enable by default so that clang
     plugin tests run by default
 - plugin_host() template