With the migration to #pragma once in the core header in #21405,
the workaround for the header name check when using #include_next
is not necessary anymore and can be dropped.
The headerguard check did not catch the case when a #pragma once
is present in the file followed by classic header guards.
This commit also improves the error messages to give a better
reason why the test failed.
When adding the #pragma once after a comment block, many editors
will remain at the previous indentation level, adding a leading
whitespace to the #pragma once. This is invalid, but causes the
headerguards check to fail. Since it is a common issue, it
warrants a separate check with a proposed solution, just like
for other headerguard issues that are checked.
