[llvm-dev] GN build roundtable summary; adding GN build files to the repo

[Petr Hosek via llvm-dev]

I'll volunteer to maintain the GN files if the community is in favor
of having those in the tree. I mentioned that I'd consider using GN
build on bots (and Nico said the same for Chromium) during the
roundtable discussion, but what I meant was for our downstream bots.
If there are upstream bots using GN, they should be FYI only bots (I
don't know if there are any such bots in LLVM already) that don't send
out any notifications on breakages.

There are other advantages to GN but those are not immediately
obvious. GN has a first-class concept of a toolchain and it allows
"applying" these toolchains on dependencies, so you can express the
fact that a particular dependency should be built with a different
toolchain, which could be a completely different compiler or just a
different set of flags. This is an extremely powerful concept that
would be useful for things like runtimes build. Right now, those
require re-running CMake multiple times for each target you want to
build runtimes for, so take the number from above responses and
multiply that by the number of targets you're supporting in your
toolchain and suddenly it may become significant (it definitely is in
our case).

Regarding the experimental status, I think we need to get the GN build
to the point where it provides parity with the CMake build and handles
all common use cases and workflows before we can have that discussion.