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gn build: Add get.py script to download prebuilt gn, make gn.py run downloaded…

Description

gn build: Add get.py script to download prebuilt gn, make gn.py run downloaded gn if gn is not on PATH

Prebuilts are available for x86_64 Linux, macOS, Windows. The script always
pulls the latest GN version.

Differential Revision: https://reviews.llvm.org/D57256