Make DisableIntegratedAS a TargetOption.

This replaces the old NoIntegratedAssembler with at TargetOption. This is
more flexible and will be used to forward clang's -no-integrated-as option.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@201836 91177308-0d34-0410-b5e6-96231b3b80d8