Add option make_clang_dir to GN
Add option make_clang_dir to GN

Same as the GYP flag with the same name. Useful for e.g. compiling with
Icecc.

BUG=None

Review URL: https://codereview.chromium.org/1610093003

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#372309}

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