Various reflect operations permit assignability conversions,
i.e. their internals behave unlike y=x.(T) which unpacks only
those interface values in x that are identical to T.
We split typeAssertConstraint y=x.(T) into two constraints:
1) typeFilter, for when T is an interface type and no
representation change occurs.
2) unpack, for when T is a concrete type and the payload of the
tagged object is extracted. This constraint has an 'exact'
parameter indicating whether to use the predicate
IsIdentical (for type assertions) or
IsAssignable (for reflect operators).
+ Tests.
R=crawshaw
CC=golang-dev
https://golang.org/cl/14547043
d7a9805478