[clang-tools-extra] [clangd][test] Add `clang` to `CLANGD_TEST_DEPS`. (PR #230061)

[Ingo Müller via cfe-commits]

ingomueller-net wrote:

FWIW, my agent proposed these two alternatives:

> 1. Have clangd itself produce %t/M.pcm in the lit test: Run the --reuse-prebuilt-module-file=false step first (which causes clangd to precompile M.cppm in-process into %t/.cache/clangd/modules/...), copy that generated .pcm file to %t/M.pcm, and then run clangd with reuse enabled.
> 2. Move the test into PrerequisiteModulesTest.cpp: TEST_F(PrerequisiteModulesTests, PrebuiltModuleFileTest) already tests prebuilt module reuse in ClangdTests by using ModulesBuilder in-process to emit M.pcm without invoking clang.

https://github.com/llvm/llvm-project/pull/230061