[ELF] Respect NonAlloc when copying flags from the previous sections

Summary:
If the output section contains only symbol assignments, we copy flags
from the previous sections. Don't set SHF_ALLOC if NonAlloc is true.

We also have to change the type from SHT_NOBITS to SHT_PROGBITS.
In ld.bfd, bfd_elf_get_default_section_type maps non-alloctable sections to SHT_PROGBITS.
Non-alloctable SHT_NOBITS sections do not make sense.

Fixes PR38626

Differential Revision: https://reviews.llvm.org/D59986

llvm-svn: 358650