[llvm-link] use file magic when deciding if input should be loaded as archive

llvm-link should not rely on the '.a' file extension when deciding if input file
should be loaded as archive. Archives may have other extensions (f.e. .lib) or no
extensions at all. This patch changes llvm-link to use llvm::file_magic to check
if input file is an archive.

Reviewed By: RaviNarayanaswamy

Differential Revision: https://reviews.llvm.org/D92376