[InputGraph][Gnu] Add LinkerScript support.

This adds LinkerScript support by creating a type Script which is of type
FileNode in the InputGraph. Once the LinkerScript Parser converts the
LinkerScript into a sequence of command, the commands are handled by the
equivalent LinkerScript node for the current Flavor/Target. For ELF, a
ELFGNULdScript gets created which converts the commands to ELF nodes and ELF
control nodes(ELFGroup for handling Group nodes).

Since the Inputfile type has to be determined in the Driver, the Driver needs
to determine the complete path of the file that needs to be processed by the
Linker. Due to this, few tests have been removed since the Driver uses paths
that doesnot exist.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@195583 91177308-0d34-0410-b5e6-96231b3b80d8
[InputGraph] Fix comment.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@195584 91177308-0d34-0410-b5e6-96231b3b80d8