[GN][NFC] Update readme example to functional command

[martell]

Summary

ninja -C out/gn check-lld is not a valid command with the current checkin.

[martell]

I really hope gn catches on and it also works with the tools layout ( i.e. llvm/tools/lld ) so I can make good use of ninja -C out/gn check-lld. :)

[thakis]

lgtm; hopefully we remember to undo this once check-lld works :-)

[thakis]

In D54840#1306533, @martell wrote:

I really hope gn catches on and it also works with the tools layout ( i.e. llvm/tools/lld ) so I can make good use of ninja -C out/gn check-lld. :)

As far as I understand, that layout will go away once the transition to the monorepo is done, so I haven't spent time on supporting it.

[thakis]

If you still want to land this, maybe use llvm-undname since that's a useful binary that now works.

llvm/trunk/utils/gn/README.rst

 
 #. 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 check-lld` to build all prerequisites for and
-   run the LLD tests.
+#. 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.
 
 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.