Set the QualType for enums, too; return 0 for cast-from-void.
Default init TInterpreterValue in TClingValue copy ctor. Wording.
Implement cheap move ctor/op=. Member-init order.
No vector for time-critical i'faces; use SmallVectorImpl in libCling i'faces.

The use of vector forces an allocation for each call, something we don't need to
impose.

SmallVectorImpl doesn't expose the static alloc size; and our interfaces don't
care. Only the actual storage definition needs to define the size, no subsequent
reference.