[LLD][ELF] - Linker script: accept using a file name without a list of sections.

This is a part of
https://bugs.llvm.org/show_bug.cgi?id=39885

Linker script specification says:
"You can specify a file name to include sections from a particular file. You would
do this if one or more of your files contain special data that needs to be at a
particular location in memory."

LLD did not accept this syntax. The patch implements it.

Differential revision: https://reviews.llvm.org/D55324

llvm-svn: 348463