build: add compile_commands.json target

Add a convenience target generating the compilation database from a
built object tree, alongside the other developer conveniences (tags,
cscope, cloc -- the last of which already walks the same .cmd files):

    make -C xen compile_commands.json

The output lands in the object tree root.  For an in-tree build,
clangd and other consumers discover it there automatically when
opening files; for an out-of-tree build, symlink it into the source
tree root (consumers search the ancestors of the file being edited).

Note the database records the compiler invocations actually used.
With a clang build it is consumable by clangd as-is; for a gcc build,
clang-based tools may need a small .clangd configuration
(CompileFlags: Remove/Add) dropping gcc-only flags.

Also add the generated file to .gitignore.

Assisted-by: LLM
Signed-off-by: George Dunlap <gwd@xenproject.org>
Reviewed-by: Anthony PERARD <anthony.perard@vates.tech>

.gitignore

@@ -192,6 +192,7 @@ xen/arch/*/include/generated
 xen/build-dir-cppcheck/
 xen/common/config_data.S
 xen/common/config.gz
+xen/compile_commands.json
 xen/cppcheck-htmlreport/
 xen/cppcheck-report/
 xen/cppcheck-misra.*

xen/Makefile

@@ -685,6 +685,9 @@ cloc:
 	    done; \
 	done | cloc --list-file=-
 
+compile_commands.json: FORCE
+	$(PYTHON) $(srctree)/scripts/gen_compile_commands.py -d $(objtree)
+
 # Target used by xen-analysis.sh script to retrieve Xen build system variables
 export-variable-%:
 	$(info $*=$($*))