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.