clang-cl: Implement support for the /Fo option

This implements support for the /Fo option, which is used
to set the filename or output dir for object files.

Differential Revision: http://llvm-reviews.chandlerc.com/D1302

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@187820 91177308-0d34-0410-b5e6-96231b3b80d8