Recognize .lib files as linker input explicitly

Summary:
Currently, this is done implicitly in Driver::BuildInputs by considering
any invalid input type as linker input.

Test Plan: I don't think this behavior is observable for the reason stated above.

Reviewers: hansw

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D5294

llvm-svn: 217522