Do not read the same .lib file more than once.

In the following command line,

  lld-link foo/bar.lib /defaultlib:bar.lib

"/defaultlib:bar.lib" should be a nop even if a file with the same
name exists in other library search path.

Fixes https://bugs.llvm.org/show_bug.cgi?id=35476

Differential Revision: https://reviews.llvm.org/D41094

llvm-svn: 320434