[ELF] Add --as-needed.

The previous default behavior of LLD is --as-needed. LLD linked
against a DSO only if the DSO file was actually used to link an
executable (i.e. at least one symbol was resolved using the shared
library file.)

In this patch I added a boolean flag to FileNode for --as-needed.
I also added an accessor to DSO name to shared library file class.

llvm-svn: 226274