[clangd] Fix test failure when it's built with compiler flags unknown by clang

If LLVM is built with a compiler other than clang, the `compile_commands.json`
file may contain compiler flags unknown by clang.  When a clangd test is copied
into the build directory and checked, clangd will pick the unknown flag from
the file and cause a test failure.  Create an empty `compile_commands.json` in
the test directory nested in the build directory to override it.

Reviewed By: thesamesam

Differential Revision: https://reviews.llvm.org/D150582