COFF: add support for lib mode usage

When link is invoked with `/def:` and no input files, it behaves as if
`lib.exe` was invoked.  Emulate this behaviour, generating the import
library from the def file that was passed.  Because there is no input to
actually generate the dll, we simply process the def file early and exit
once we have created the import library.

llvm-svn: 305502