create compile commands for each file when unity build

When a target is build with unity build, only generated unity_n_cxx.cxx file’s compile command are exported in the compile_commands.json , but the raw source files are ignored . actually many c/c++ language server need the original source file’s compile commands information, and the programmer also need them when work in IDE, so , I think it’s necessary to generate all source files compile commands in the result json file.

See issue 19826 which discusses this limitation.