[perl #108994] Stop SvPVutf8 from coercing SVs

[perl #108994] Stop SvPVutf8 from coercing SVs

In shouldn’t destroy globs or references passed to it, or try to
coerce them if they are read-only or incoercible.

I added tests for SvPVbyte at the same time, even though it was not
exhibiting the same problems, as sv_utf8_downgrade doesn’t try to
coerce anything.  (SvPVbyte has its own set of bugs, which I hope to
fix in fifthcoming commits.)