[NFC][clang-tools-extra] Inclusive language: replace master with main

[NFC] As part of using inclusive language within the llvm project, this patch
replaces master with main in `SubModule2.h`.

Reviewed By: sammccall

Differential Revision: https://reviews.llvm.org/D114100

GitOrigin-RevId: c3dc6b081da6ba503e67d260033f81f61eb38ea3
1 file changed