Issues:

• On Windows, the Arduino-generated compile_commands.json may be missing the .exe extension, which causes clangd to not be able to query the compiler driver. postbuild.js script appends the extension for now.
• If using gcc-arm-none-eabi-7-2017-Q4 or earlier, there is a bug where the extracted system includes (which are passed to the compiler via the -isystem flag) will have C linkage instead of C++ linkage, which causes a lot of compiler warnings, especially through clangd intellisense... ** A drop-in replacement of gcc-arm-none-eabi-10-2020-q4-major into the toolchain folder fixes this issue... or you can wait for Arduino to update the toolchain.