clang-cl: add support for the /? and /help options

This establishes a new Flag in Options.td, which can be assigned to
options that should be made available in clang's cl.exe compatible
mode, and updates the Driver to make use of the flag.

(The whitespace change to CMakeLists forces the build to re-run CMake
 and pick up the include dependency on the new .td file. This makes the
 build work if someone moves backwards in commit history after this change.)

Differential Revision: http://llvm-reviews.chandlerc.com/D1215