clang-cl: parse (but don't support) the /o flag

Apparently folks run into this (PR17097). The flag is not supported by
MSVC either, but we should parse it so we don't get confused when it occurs.

This changes the clang-cl output for "clang-cl /c /o foo.obj" from:

  clang-cl.exe: error: no such file or directory: '/o'
  clang-cl.exe: error: no such file or directory: 'foo.obj'

to:

  clang-cl.exe: warning: argument unused during compilation: '/o bajs.obj'

llvm-svn: 190323