Disallow using -fmodules with -no-integrated-as.

Modules enables features such as auto-linking, and we simply do not want to
support a matrix of subtly enabled/disabled features depending on whether or
not a user is using the integrated assembler.

It isn't clear if this is the best place to do this check.  For one thing,
these kind of errors are not caught by the serialized diagnostics.

Fixes <rdar://problem/13289240>