[ELF] Lookup INPUT argument in the current directory

If an argument of the INPUT directive is a regular path, linker should
lookup it in the current folder first.

The fix does not contain any test cases because I think it is not a good
idea to pollute a current folder (which in general might be arbitrary)
by test files.

Differential Revision: http://reviews.llvm.org/D15027

llvm-svn: 254178