Removed ReadVal from SerializeTrait<T>, and also removed it from

Removed ReadVal from SerializeTrait<T>, and also removed it from
Deserializer.

There were issues with Visual C++ barfing when instantiating
SerializeTrait<T> when "T" was an abstract class AND
SerializeTrait<T>::ReadVal was *never* called:

template <typename T>
struct SerializeTrait {
 <SNIP>
  static inline T ReadVal(Deserializer& D) { T::ReadVal(D); }
 <SNIP>
};

Visual C++ would complain about "T" being an abstract class, even
though ReadVal was never instantiated (although one of the other
member functions were).

Removing this from the trait is not a big deal.  It was used hardly
ever, and users who want "read-by-value" deserialization can simply
call the appropriate methods directly instead of relying on
trait-based-dispatch.  The trait dispatch for
serialization/deserialization is simply sugar in many cases (like this
one).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@43624 91177308-0d34-0410-b5e6-96231b3b80d8