A character in a fixed-form source line (as opposed to a comment or
compiler directive) can't have a non-digit in its label field, columns 1
through 5. The prescanner presently emits only a warning for this case.
Retain the warning for -E output, but otherwise diagnose an error.
(This change affected a number of tests that relied on this situation
being a warning just so that they could test prescanner warnings, and
those tests were adjusted to use another warning.)
Fixes https://github.com/llvm/llvm-project/issues/50563.
b4f9012464