Fix an assertion failure when invoking dsymutil.

There is no bound architecture for the dsymutil action in the driver. Trying
to check various properties of the target will cause an assertion failure
because the target doesn't get initialized without a bound architecture.
<rdar://problem/16111555>

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@201830 91177308-0d34-0410-b5e6-96231b3b80d8