[Driver] Replace Target with TargetInfo. Simplify LinkerInput.

This removes Target and moves the functionality it had over to TargetInfo.

This also simplifies LinkerInput by removing the InputKind. This will be handled elsewhere.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@174589 91177308-0d34-0410-b5e6-96231b3b80d8