[ELF] - Do not crash when LLD synthesizes output sections with BYTE commands and -r

This is PR33596. Previously LLD would crash
because BYTE command synthesized output section,
but it was not assigned to Sec member of OutputSectionCommand.

Behaviour of -script and -r combination is not well defined,
but it seems after this change LLD naturally inherits behavior of
GNU linkers - creates output section requested in script and does not
crash anymore.

Differential revision: https://reviews.llvm.org/D34676

llvm-svn: 306527