Change `DECL_PRE_FN`/`DECL_POST_FN` visibility, COMDAT section

Since these are currently internal implementation details they should be made hidden and put in the same COMDAT section with their guarded function similar to constructor variants. In that case we don't need to worry much about the specifics of mangling.

We should add a note that these may need to be standardized through the ABI later due to potential extensions.
