Reject .so files if -static is given.

Previously, if you pass -static to lld, lld searches for only foo.a
and skips foo.so for -lfoo option. However, it didn't reject .so files
if you directly pass their pathnames via the command line, which is a bug.

Differential Revision: https://reviews.llvm.org/D55845

llvm-svn: 349557