[gn build] Add a TODO.txt file

[thakis]

Summary

Probably makes sense to keep this in a checked-in file instead of in a file on my local disk.

[phosek]

Can we make it .rst or .md so it's formatted nicely when opened e.g. on GitHub?

[thakis]

I'd prefer to land this as a txt file:

• rst requires a newline after toplevel list entries to accept nested lists, which makes the txt file less readable
• md doesn't, but it insists on `` around every string that contains an underscore, else parts of the word become italic

That is, editing the file gets much more annoying, and the file is reasonably readable when rendered as txt too.

Sound ok?

[phosek]

I'd still prefer Markdown, but I'm also used to writing things in Markdown so I don't see that as a downside personally. However, I think it's a minor thing so I'm not going to push for it ;)

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
+  - once there are more projects, have an llvm_enable_projects arg, modeled
+    after llvm_targets_to_build in the GN build
+- 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 write_cmake_config() template, use it throughout where
+  write_cmake_config.py is called
+- add dead code stripping
+- move run_tablegen.py from build to tablegen folder
+- figure out why -Iclang/Support gets added so often
+- make LLVM_LIBXML2_ENABLED use llvm_canonicalize_cmake_boolean (cf D28294),
+  clean up both GN and CMake builds
+- 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