[clangd] Make tests depend on Clang

[smeenai]

Summary

The clangd lit configuration adds a depedency on Clang, so we also need
to add a build dependency to make check-clangd work when you start
from a clean build.

[sammccall]

This would be pretty unfortunate for iteration times: in my workspace having just run ninja check-clangd, ninja clang has 1450 additional build steps remaining. (All of which would get frequently invalidated e.g. by changes to common LLVM headers).

clangd/test/lit.cfg.py mentions clang, our intent here is configure some common required variables etc, not depend clang itself. It's not very well-defined though :-(
I'm pretty sure it used to work at some point, but now after a clean:

llvm-lit: /usr/local/google/home/sammccall/src/llvm-project/llvm/utils/lit/lit/llvm/config.py:342: fatal: couldn't find 'clang' program, try setting CLANG in your environment

Looking at the contents of llvm/config.py, it's getting the builtin includes directory in order to use it in lit substitutions like %clang_cc1. Which of course we don't use in clangd tests.
I'm trying to remember exactly why we do need to use this module at all, but I suspect there was a real reason.

I think we're going to need to refactor llvm/config.py a little to let us take the bits we want. I can have a dig into this soon...

[sammccall]

D90528 should solve this in a cheaper way. Thanks for raising this though!

@kbobyrev @kadircet we were thinking about adding a buildbot with grpc/remote index enabled. Maybe we should have that one build check-clangd only, and not clang?
It would be a little faster and it would ensure this configuration doesn't break in future.

[smeenai]

Cool, thanks for fixing it better!

clang-tools-extra/clangd/test/CMakeLists.txt

 # Set CLANG_TOOLS_DIR to buildtree/bin, or buildtree/%(build_mode)s/bin if the
 # location is dynamic. The latter must be interpolated by lit configs.
 # FIXME: this is duplicated in many places.
 if (CMAKE_CFG_INTDIR STREQUAL ".")
   set(LLVM_BUILD_MODE ".")
 else ()
   set(LLVM_BUILD_MODE "%(build_mode)s")
 endif ()
 string(REPLACE ${CMAKE_CFG_INTDIR} ${LLVM_BUILD_MODE} CLANG_TOOLS_DIR ${LLVM_RUNTIME_OUTPUT_INTDIR})
 
 set(CLANGD_TEST_DEPS
+  clang
   clangd
   ClangdTests
   # No tests for these, but we should still make sure they build.
   clangd-indexer
   dexp
   )
 
 if(CLANGD_BUILD_XPC)