llvm_tools: Add GitHub API usage to get_patch.py

This commit allows get_patch.py to pull down PRs from GitHub,
and convert them to patches inside PATCHES.json.

BUG=b:318755454
TEST=run_tests_for.py llvm_tools/get_patch.py

Change-Id: Iebbe2671f0917f808e0012393498d09910809831
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/third_party/toolchain-utils/+/5171748
Tested-by: Jordan Abrahams-Whitehead <ajordanr@google.com>
Commit-Queue: Jordan Abrahams-Whitehead <ajordanr@google.com>
Reviewed-by: George Burgess <gbiv@chromium.org>
1 file changed