[clangd] Add documentation for building and testing clangd

[kadircet]

Summary

Adds minimal cmake configuration required to build and test clangd,
while telling target names. Should be helpful for people unfamiliar with the
LLVM repo.

See https://github.com/clangd/clangd/issues/579 for a request.

[nridge]

clang-tools-extra/clangd/README.md

30

I guess it might depend on whether you just want to build from source, or if you're doing clangd development. For development purposes, I find release builds to be very hard to debug due to inlining and other optimizations.

[kadircet]

clang-tools-extra/clangd/README.md

30

yeah i suppose it might make sense to add something like CXX_FLAGS="-gmlt -O1" but I am not sure if we mention that in this minimal one :/

[sammccall]

Thanks!

clang-tools-extra/clangd/README.md

26

this will work, though won't it use make by default with no parallelism?

If we're recommending more flags, i'd suggest -DLLVM_ENABLE_ASSERTS=On, and -G Ninja...

27

nit: traditionally Release rather than RELEASE (though IDK if it matters)

[kadircet]

• Mention assertions and ninja

clang-tools-extra/clangd/README.md

26

i wanted to keep it to a bare minimum, hence no mentions of ninja and asserts. i am adding a paragraph below to suggest adding those.

[nridge]

I wonder, should these instructions be in an easier-to-find place, such as the clangd repo's own README (https://github.com/clangd/clangd/blob/master/README.md), or even a section of the clangd website (clangd.llvm.org)?

I wanted to send build instructions to someone who was asking for them, and it took me a while to find this (had to come back and find this review request to see what page was ultimately edited here).

clang-tools-extra/clangd/README.md

 
 If you have any questions or feedback, you can reach community and developers
 through one of these channels:
 
 - chat: #clangd room hosted on [LLVM's Discord
   channel](https://discord.gg/xS7Z362).
 - user questions and feature requests can be asked in the clangd topic on [LLVM
   Discussion Forums](https://llvm.discourse.group/c/llvm-project/clangd/34)
+
+### Building and testing clangd
+
+For a minimal setup on building clangd:
+- Clone the LLVM repo to `$LLVM_ROOT`.
+- Create a build directory, for example at `$LLVM_ROOT/build`.
+- Inside the build directory run: `cmake $LLVM_ROOT/llvm/
+  -DCMAKE_BUILD_TYPE=Release -DLLVM_ENABLE_PROJECTS="clang;clang-tools-extra"`.
+
+  - We suggest building in `Release` mode as building DEBUG binaries requires
+    considerably more resources. You can check
+    [Building LLVM with CMake documentation](https://llvm.org/docs/CMake.html)
+    for more details about cmake flags.
+  - In addition to that using `Ninja` as a generator rather than default `make`
+    is preferred. To do that consider passing `-G Ninja` to cmake invocation.
+  - Finally, you can turn on assertions via `-DLLVM_ENABLE_ASSERTS=On`.
+
+- Afterwards you can build clangd with `cmake --build $LLVM_ROOT/build --target
+  clangd`, similarly run tests by changing target to `check-clangd`.