Report an error if you try to link against .dll instead of .lib.

It is a usage error to feed a .dll file instead of a .dll to COFF linker.
Previously, lld failed with a mysterious error message. Now we reject
it at the driver.

Fixes https://bugs.llvm.org/show_bug.cgi?id=36440

Differential Revision: https://reviews.llvm.org/D43964

llvm-svn: 326507