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To consume win32-specific cmdline parameters (e.g. -imsvc, /FI or /Yu)
clang needs to be invoked with `--driver-mode=cl`.  This CL ensures that
the driver-mode cmdline switch is appended not only when running a
rewriter tool on Windows, but also when run_tool.py or
generate_compdb.py are invoked with an explicit --target_os=win cmdline
argument.

Manual tests:

1. Manually run unit tests for tools/clang/pylib/clang/compile_db.py:
    $ pushd tools/clang/pylib/clang/
    $ python compile_db_test.py

2. Manually tried running the rewriter on linux, when targeting win:
    $ cat out/rewrite-win/args.gn
    clang_use_chrome_plugins = false
    target_os = "win"
    $ tools/clang/scripts/run_tool.py --tool rewrite_raw_ptr_fields \
        --generate-compdb --target_os=win -p out/rewrite-win \
        content/browser/renderer_host/render_process_host_impl.cc \
        >~/scratch/rewriter.out

3. Manually tried running generate_compdb.py:
    $ tools/clang/scripts/generate_compdb.py -p out/rewrite gn_all \
        -o ~/scratch/compdb

Bug: 1069567
Change-Id: I54fcee164d7bb05119c8267b43e0d872bc2f436b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2481745
Commit-Queue: Łukasz Anforowicz <lukasza@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#818687}