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

This removes a bunch of llvm_unreachables and makes a File's mutability part of the type system.

llvm-svn: 172297

[Archive] Use a hash map to lookup symbols in archives. Increases performance of linking lua by ~6x.

llvm-svn: 172028

llvm-svn: 167871

llvm-svn: 167869

llvm-svn: 167854