scripts/clang-tools: Handle included .c files in gen_compile_commands

scripts/clang-tools: Handle included .c files in gen_compile_commands

The gen_compile_commands.py script currently only creates entries for the
primary source files found in .cmd files, but some kernel source files
text-include others (i.e. kernel/sched/build_policy.c).

This prevents tools like clangd from working properly on text-included c
files, such as kernel/sched/ext.c because the generated compile_commands.json
does not have entries for them.

Extend process_line() to detect when a source file includes .c files, and
generate additional compile_commands.json entries for them. For included c
files, use the same compile flags as their parent and add their parents headers.

This enables lsp tools like clangd to work properly on files like
kernel/sched/ext.c

Signed-off-by: Pat Somaru <patso@likewhatevs.io>
Reviewed-by: Nathan Chancellor <nathan@kernel.org>
Tested-by: Justin Stitt <justinstitt@google.com>
Tested-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://patch.msgid.link/20251008004615.2690081-1-patso@likewhatevs.io
Signed-off-by: Nicolas Schier <nsc@kernel.org>