llvm-svn: 177079

I really would have liked to split this patch up, but it would greatly
complicate the lld-core and lld drivers having to deal with both
{Reader,Writer}Option and TargetInfo.

llvm-svn: 173217

now Reader and Writer subclasses for each file format.  Each Reader and
Writer subclass defines an "options" class which controls how that Reader
or Writer operates.

llvm-svn: 157774