[clang-tidy] If we're going to change the header guard in the #endif comment
we can also fix the original header guard.

We still allow an _ at the end of a header guard since it's so common, but
remove it now when the #endif comment is changed.

git-svn-id: https://llvm.org/svn/llvm-project/clang-tools-extra/trunk@216462 91177308-0d34-0410-b5e6-96231b3b80d8