[PECOFF] Treat .imp as an import library file.

Some Clang build uses .imp not .lib file extension for an import library file,
so we need to treat such file as a library file.

Ideally we should not rely on file extensions to detect file type. Instead
we should use magic bytes at beginning of a file. The GNU-compatible driver
actually does that but it made writing unit tests hard, so I chose an ad-hoc
approach for now.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@205283 91177308-0d34-0410-b5e6-96231b3b80d8