[ELF] - Do not collect SHT_REL[A] sections unconditionally when --gc-sections...

[ELF] - Do not collect SHT_REL[A] sections unconditionally when --gc-sections and --emit-relocs used together.

This is "Bug 34836 - --gc-sections remove relocations from --emit-relocs",

When --emit-relocs is used, LLD currently always drops SHT_REL[A] sections from
output if --gc-sections is present. Patch fixes the issue.

Differential revision: https://reviews.llvm.org/D38724

llvm-svn: 316418