llvm_tools: update upstream branch names to point to `main`

LLVM recently renamed their `master` branch to `main` upstream. This
changes our tooling to search for the latter name instead.

BUG=None
TEST=Unittests; ran `git llvm-rev --sha HEAD` in an up-to-date llvm
repo.

Change-Id: Icdc6dac48fda7697d18d0d77935e46670273bf84
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/third_party/toolchain-utils/+/2577643
Reviewed-by: Manoj Gupta <manojgupta@chromium.org>
Tested-by: George Burgess <gbiv@chromium.org>
2 files changed