[OpenACC] device_type clause Sema for Compute constructs

device_type, also spelled as dtype, specifies the applicability of the
clauses following it, and takes a series of identifiers representing the
architectures it applies to.  As we don't have a source for the valid
architectures yet, this patch just accepts all.

Semantically, this also limits the list of clauses that can be applied
after the device_type, so this implements that as well.