Using a StringInfo just to copy a string is quite pointless. Simplify by
changing OptVersion() to return a plain palloc'd string instead.

This fixes a memory management bug too: OptVersion() is called like a
normal Postgres C function, not as a subroutine of PplStmtOptimize. As a
result, if OptVersion() throws a C++ exception, there is nothing to catch
it, and it will cause the process to exit, bringing down the server.
The gpdb::SiMakeStringInfo() wrapper, used in OptVersion(), would translate
any ereport() (e.g. out-of-memory error) into a C++ error, but that's not
what we want in this context. A plain makeStringInfo() would be correct
here, and LibraryVersion() got that right, but for OptVersion it's simpler
to just return a plain string anyway.