Unmangle names consistently when using --allow-redefinition

[JukkaL]

I expect that some error message will sometimes contain unmangled variable names with prime suffixes (such as foo'). We should review all error message generation code and add unmangle() calls as needed. This is not super urgent since a single extra prime is not visually very prominent, but it's still annoying when it happens.

This is a follow-up issue to #6197 (see the PR for examples of what needs to be done).