This is step 2 towards fixing #77548.
In the codegen and codegen-units test suites, the `//` comment markers
were kept in order not to affect any source locations. This is because
these tests cannot be automatically `--bless`ed.
it uses normalize-stderr-test because not all targets hit the same OS error number nor message ...
... and ignores tidy since I dont know how to make the normalize line shorter ...
and has effectively a no-op for its error-pattern because the targets' error
messages are so wildly different (and the error-pattern check occurs *before*
stderr normalization.)
