[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

(cherry picked from commit 4ddae8b)

Author: xry111

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

@@ -28,6 +28,16 @@ configure_lit_site_cfg(
   ${CMAKE_CURRENT_SOURCE_DIR}/lit.cfg.py
   )
 
+# Copy an empty compile_commands.json to override the compile_commands.json
+# in the top level build directory.  Or if a clangd test involves creating a
+# temporary source file in the build directory and run clangd to check it,
+# it can pick up unrecognizable command options when LLVM is built with
+# another compiler or a different version of Clang.
+configure_file(
+  ${CMAKE_CURRENT_SOURCE_DIR}/compile_commands.json
+  ${CMAKE_CURRENT_BINARY_DIR}/compile_commands.json
+)
+
 add_lit_testsuite(check-clangd "Running the Clangd regression tests"
   # clangd doesn't put unittest configs in test/unit like every other project.
   # Because of that, this needs to pass two folders here, while every other

clang-tools-extra/clangd/test/compile_commands.json

@@ -0,0 +1 @@
+[]