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.

TEST=Unittests; ran `git llvm-rev --sha HEAD` in an up-to-date llvm

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>
diff --git a/llvm_tools/get_llvm_hash.py b/llvm_tools/get_llvm_hash.py
index 4c47996..329e829 100755
--- a/llvm_tools/get_llvm_hash.py
+++ b/llvm_tools/get_llvm_hash.py
@@ -293,7 +293,7 @@
   def GetTopOfTrunkGitHash(self):
     """Gets the latest git hash from top of trunk of LLVM."""
-    path_to_main_branch = 'refs/heads/master'
+    path_to_main_branch = 'refs/heads/main'
     llvm_tot_git_hash = check_output(
         ['git', 'ls-remote', _LLVM_GIT_URL, path_to_main_branch])
     return llvm_tot_git_hash.rstrip().split()[0]
diff --git a/llvm_tools/git_llvm_rev.py b/llvm_tools/git_llvm_rev.py
index 07209f1..c8c1505 100755
--- a/llvm_tools/git_llvm_rev.py
+++ b/llvm_tools/git_llvm_rev.py
@@ -7,7 +7,7 @@
 """Maps LLVM git SHAs to synthetic revision numbers and back.
 Revision numbers are all of the form '(branch_name, r1234)'. As a shorthand,
-r1234 is parsed as '(master, 1234)'.
+r1234 is parsed as '(main, 1234)'.
 from __future__ import print_function
@@ -18,7 +18,7 @@
 import sys
 import typing as t
-MAIN_BRANCH = 'master'
+MAIN_BRANCH = 'main'
 # Note that after base_llvm_sha, we reach The Wild West(TM) of commits.
 # So reasonable input that could break us includes: