clang-cl: implement /fallback mode

When this flag is enabled, clang-cl falls back to cl.exe if it
cannot compile the code itself for some reason.

The idea is to use this to help build projects that almost compile
with clang-cl, except for some files that can then be built with
the fallback mechanism.

Differential Revision: http://llvm-reviews.chandlerc.com/D1711

llvm-svn: 191034