[ELF] - Allow LLD to produce file symbols.

This is for PR36716 and
this enables emitting STT_FILE symbols.

Output size affect is minor:
lld binary size changes from 52,883,408 to 52,949,400
clang binary size changes from 83,136,456 to 83,219,600

Differential revision: https://reviews.llvm.org/D45261

llvm-svn: 329557