clang-cl: Add support for the /o option for object files, executables, and preprocessor output

Summary:
cl.exe recognizes /o as a deprecated and undocumented option similar to
/Fe.  This patch adds support for this option to clang-cl for /Fe, /Fo
and /Fi.  It also ensures that the last option among /o and /F* wins,
if both specified.

This is required at least for building autoconf based software, since
autoconf uses -o to specify the executable output.

This fixes http://llvm.org/PR20894.

Test Plan: The patch includes automated tests.

Reviewers: rnk

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D5308