[clang-tidy] Reword the "code outside header guard" warning.

The check doesn't really know if the code it is warning about came before
or after the header guard, so phrase it more neutral instead of complaining
about code before the header guard. The location for the warning is still
not optimal, but I don't think fixing that is worth the effort, the
preprocessor doesn't give us a better location.

Differential Revision: https://reviews.llvm.org/D30191

llvm-svn: 295715