[NFC][gn build] Inclusive language: replace master with main in sync_source_lists_from_cmake.py

[quinnp]

Summary

[NFC] As part of using inclusive language within the llvm project and to
match the renamed master branch, this patch replaces master with main in
sync_source_lists_from_cmake.py.

[thakis]

Thanks!

llvm/utils/gn/build/sync_source_lists_from_cmake.py

 #!/usr/bin/env python
 
 """Helps to keep BUILD.gn files in sync with the corresponding CMakeLists.txt.
 
 For each BUILD.gn file in the tree, checks if the list of cpp files in
 it is identical to the list of cpp files in the corresponding CMakeLists.txt
 file, and prints the difference if not.
 
 Also checks that each CMakeLists.txt file below unittests/ folders that define
 binaries have corresponding BUILD.gn files.
 
 If --write is passed, tries to write modified .gn files and adds one git
 commit for each cmake commit this merges. If an error is reported, the state
-of HEAD is unspecified; run `git reset --hard origin/master` if this happens.
+of HEAD is unspecified; run `git reset --hard origin/main` if this happens.
 """
 
 from __future__ import print_function
 
 from collections import defaultdict
 import os
 import re
 import subprocess