The LLVM/Clang project provides many tools for analyzing C source code.
Many of these tools are based on LibTooling
(https://clang.llvm.org/docs/LibTooling.html), which depends on a
database of compiler flags. The standard container for this database is
compile_commands.json, which consists of a list of JSON objects, each
with "directory", "file", and "command" fields.

Some build systems, like cmake or bazel, produce this compilation
information directly. Naturally, Makefiles don't. However, the kernel
makefiles already create .<target>.o.cmd files that contain all the
information needed to build a compile_commands.json file.

So, this commit adds scripts/gen_compile_commands.py, which recursively
searches through a directory for .<target>.o.cmd files and extracts
appropriate compile commands from them. It writes a
compile_commands.json file that LibTooling-based tools can use.

By default, gen_compile_commands.py starts its search in its working
directory and (over)writes compile_commands.json in the working
directory. However, it also supports --output and --directory flags for
out-of-tree use.

Note that while gen_compile_commands.py enables the use of clang-based
tools, it does not require the kernel to be compiled with clang. E.g.,
the following sequence of commands produces a compile_commands.json file
that works correctly with LibTooling.

make defconfig
make
scripts/gen_compile_commands.py

Also note that this script is written to work correctly in both Python 2
and Python 3, so it does not specify the Python version in its first
line.

For an example of the utility of this script: after running
gen_compile_commands.json on the latest kernel version, I was able to
use Vim + the YouCompleteMe pluging + clangd to automatically jump to
definitions and declarations. Obviously, cscope and ctags provide some
of this functionality; the advantage of supporting LibTooling is that it
opens the door to many other clang-based tools that understand the code
directly and do not rely on regular expressions and heuristics.

Tested: Built several recent kernel versions and ran the script against
them, testing tools like clangd (for editor/LSP support) and clang-check
(for static analysis). Also extracted some test .cmd files from a kernel
build and wrote a test script to check that the script behaved correctly
with all permutations of the --output and --directory flags.
Signed-off-by: Tom Roeder <tmroeder@google.com>
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>