[ELF] Remove {ELF,}GNULinkerScript.

Instead of representing a linker script file as an "InputElement",
parse and evaluate scripts in the driver as we see them.

Linker scripts are not regular input files (regular file is one of
object, archive, or shared library file). They are more like
extended command line options. Linker script handling was needlessly
complicated because of that inappropriate abstraction (besides
excessive class hierarchy -- there is no such thing like ELF linker
script but we had two classes there for some reason.)

LinkerScript was one of a few remaining InputElement subclasses
that can be expanded to multiple files. With this patch, we are one
step closer to retire InputElement.

http://reviews.llvm.org/D6648

llvm-svn: 225330