[Driver] Don't forward source file input args to gcc

gcc's inputs are already added by the InputInfoList passed to
Action::ConstructJob.

Fixes a regression from r183989.  This was manifesting when targetting
mingw as an extra input argument to gcc when assembling.  It presumably
affects other situations where clang calls gcc.

Prior to r183989, forwardToGCC() was returning false because the INPUT
option defined in OptParser.td had the DriverOption flag set on it.
LLVM's Option library does not set this flag for INPUT.

Reviewers: espindola

Differential Revision: http://llvm-reviews.chandlerc.com/D999