clang-cl: Don't warn on /bigobj flag

LLVM r217812 made it so that clang-cl implicitly creates bigobj files when
needed, independent of this flag. It looks like cl has this flag to produce obj
flags compatible with MSVS 2003's linker by default, something we don't care
about. Since clang-cl always has /bigobj behavior, don't warn that the flag is
unused, just ignore it silently.

llvm-svn: 248034