[gn build] Update readme

[phosek]

LGTM

llvm/trunk/utils/gn/README.rst

 *Warning* The GN build is experimental and best-effort. It might not work,
 and if you use it you're expected to feel comfortable to unbreak it if
 necessary. LLVM's official build system is CMake, if in doubt use that.
 If you add files, you're expected to update the CMake build but you don't need
 to update GN build files. Reviewers should not ask authors to update GN build
 files. Keeping the GN build files up-to-date is on the people who use the GN
 build.
 
-*Another Warning* Right now, we're in the process of getting the GN build
-checked in. As of this writing, it's not yet functional at all. Check back
-in a few weeks!
-
-`GN <https://gn.googlesource.com/gn/>`_ is another metabuild system. It always
+`GN <https://gn.googlesource.com/gn/>`_ is a metabuild system. It always
 creates ninja files, but it can create some IDE projects (MSVC, Xcode, ...)
 which then shell out to ninja for the actual build.
 
 Its main features are that GN is very fast (it currently produces ninja files
 for LLVM's build in 35ms on the author's laptop, compared to 66s for CMake) --
 a 2000x difference), and since it's so fast it doesn't aggressively cache,
 making it possible to switch e.g. between release and debug builds in one build
 directory.
 
-It is arguable easier to configure than the CMake build, and has native support
-for building with multiple toolchains in one build directory. The build
-description is declarative-ish, allowing GN to print it in a json format that
-can fairly easily be converted to other metabuild system inputs.
-
 The main motivation behind the GN build is that some people find it more
 convenient for day-to-day hacking on LLVM than CMake. Distribution, building
 just parts of LLVM, and embedding the LLVM GN build from other builds are a
 non-goal for the GN build.
 
 This is a `good overview of GN <https://docs.google.com/presentation/d/15Zwb53JcncHfEwHpnG_PoIbbzQ3GQi_cpujYwbpcbZo/edit#slide=id.g119d702868_0_12>`_.
 
 .. _Quick start:
 
 Quick start
 ===========
 
-*Warning* Right now, we're in the process of getting the GN build checked in.
-As of this writing, it's not yet functional at all.
-
 GN only works in the monorepo layout.
 
 #. Obtain a `gn binary <https://gn.googlesource.com/gn/#getting-started>`_.
 
 #. In the root of the monorepo, run
    `gn gen --dotfile=$PWD/llvm/utils/gn/.gn --root=. out/gn` (`out/gn` is the
    build directory, it can have any name, and you can have as many as you want,
    each with different build settings).
 
-#. Run e.g. `ninja -C out/gn llvm-undname` to build all prerequisites for and
-   including the Microsoft symbol name pretty printing tool llvm-undname.
+#. Run e.g. `ninja -C out/gn check-lld` to build all prerequisites for and
+   run the LLD tests.
 
 By default, you get a release build with assertions enabled that targets
 the host arch. You can set various build options by editing `out/gn/args.gn`,
 for example putting `is_debug = true` in there gives you a debug build. Run
 `gn args --list out/gn` to see a list of all possible options. After touching
 `out/gn/args.gn`, just run ninja, it will re-invoke gn before starting the
 build.