[NFC][clang] Inclusive language: replace master with main in convert_arm_neon.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
convert_arm_neon.py.

[kristof.beyls]

LGTM, thanks.

clang/utils/convert_arm_neon.py

 #!/usr/bin/env python3
 
 # This script was committed on 20/11/2019 and it would probably make sense to remove
 # it after the next release branches.
 
 # This script is pipe based and converts an arm_neon.td (or arm_fp16.td) file
 # using the old single-char type modifiers to an equivalent new-style form where
 # each modifier is orthogonal and they can be composed.
 #
-# It was used to directly generate the .td files on master, so if you have any
+# It was used to directly generate the .td files on main, so if you have any
 # local additions I would suggest implementing any modifiers here, and running
 # it over your entire pre-merge .td files rather than trying to resolve any
 # conflicts manually.
 
 import re, sys
 MOD_MAP = {
     'v': 'v',
     'x': 'S',