Add SharedLibraryFile and ArchiveLibraryFile subclasses of File.

Add command line options to lld-core to set various ResolverOptions settings
and added lots of test cases to verify the options work.

llvm-svn: 155183

class.  Change Resolver to no longer use Platform.  Core linking
now issues errors directly.  We need to factor that out later.

Rework how Darwin executable writer finds "main" atom.  It now
adds to core linking an Atom  which has a Reference to "main".  

llvm-svn: 155060

llvm-svn: 149962