[lli] Fix crash with --no-process-syms on MinGW (#151386)
In this case, `J->getProcessSymbolsJITDylib()` returns a NULL pointer.
In order to make sure `__main` is still defined, add the symbol to
`J->getMainJITDylib()` instead in that case. This returns a reference
and thus cannot be NULL.
Fixes #143080
