Make "process attach -c" work correctly, and add a test for it.

The issue here was that we were not updating the interpreter's
execution context when calling HandleCommand to continue the process.
Since we had just created the process, it wasn't in the interpreter's
execution context so HandleCommand failed at CheckRequirements.  The
patch fixes that by passing the process execution context directly
to HandleCommand.

Differential Revision: https://reviews.llvm.org/D110787