Encode patch-version in the prefix of cxx-generated symbols.

[anforowicz]

Fixes #1507

[nyurik]

should the symbol name be something more descriptive to explain to the user what they are doing wrong?

[anforowicz]

should the symbol name be something more descriptive to explain to the user what they are doing wrong?

I don't think so - the prefix will be present in each cxx-generated symbol. If the prefix is too long, then the symbols may get unwieldy to use (e.g. when reading them on a limited-width terminal, or when writing them to set breakpoints or to describe bugs).

[dtolnay]

Thanks!