[Frontend] Fix an issue where a quoted search path is incorrectly

removed as a duplicate header search path

The commit r126167 started passing the First index into RemoveDuplicates, but
forgot to update 0 to First in the loop that looks for the duplicate. This
resulted in a bug where an -iquoted search path was incorrectly removed if you
passed in the same path into -iquote and more than one time into -isystem.

rdar://23991350

Differential Revision: https://reviews.llvm.org/D27298

llvm-svn: 288491