Respect supportsMemoryReferences in DAP

Respect supportsMemoryReferences in DAP

I noticed that the support for memoryReference in the "variables"
output is gated on the client "supportsMemoryReferences" capability.

This patch implements this and makes some other changes to the DAP
memory reference code:

* Remove the memoryReference special case from _SetResult.
  Upstream DAP fixed this oversight in response to
  https://github.com/microsoft/debug-adapter-protocol/issues/414

* Don't use the address of a variable as its memoryReference -- only
  emit this for pointer types.  There's no spec support for the
  previous approach.

* Use strip_typedefs to handle typedefs of pointers.